This spikes my curiousty too. BM remarried last year. According to her social media and email she changed her last name to current husband. 
 

I think this was out of spite but, few months ago her and I got an email from one of SDs teachers. I'll add that BM called the schools two years ago and had me taken off emails for the boys (when court started to asset control.) She never called the elementary school so my email rolled over for SD. 
 

Normally she fowards these emails to SO with "this shouldn't  have gone to her."  This time though she responded to the teacher and me (like she's involved) and she signed it With SOs last name. I laughed because her emailed showed her new last name. Like I said, pretty sure that was just a jab at me. 

The older one gets, it seems the more things there are to change your name on. I changed my name with my first marriage and I don't remember it being too hard. I had just graduated from college. I got my professional license issued in my new name. I closed the bank account I had and opened a joint with new husband. It was so long ago that credit cards were uncommon and we didn't have one. 

Second time was a huge pain. License, passport, professional license, car registration, life insurance policies, and on and on. I would never do it again. 
 

I kept ExH last name because DS asked me too. I was not happy. I am in the process of changing my name to DHs as we married in February. It is not so easy. 

Maybe she really liked the last name, or maybe she still has unrequited feelings for your DH. Who knows, who cares, he's with you now and she can go suck it.

ETA, I changed my last name with my ex and I hated all the time and effort it took to do so, then when we divorced all the time and effort changing it back. When I married DH I refused to change my name, DH was not happy and had a big man fit about it. BS has my last name and when BD was born I made my last name one of her middle names. So both my kids will carry my name no matter what. DH who was so adamant that women must change their name only has daughters and if/when Spawn marries she will change her name and if/when BD marries she will probably change hers as well, but she will always have my name. Kind of my way of sticking it to my DH for being a chauvinistic pig all those years ago.

BM here was married prior to DH, had 2 children - she took his name. When she divorced, she went back to her birth name, so she and older two kids had different last names.

Then she married DH and changed it to his name. When they divorced, she kept his name, so she and SS have the same name, but it's different from her older kids. I alway wondered why, and I'm guessing it's because she didn't want to have to change it professionally and let everyone know she divorced. She claims to feel so abused and victimized by DH but keeps his name? Weird.

I kept my own name. No way in hell am I sharing a name with BM and SS (among other reasons for not changing it).

I did not change my name after the divorce for a few reasons. I was pretty young at the time and the few people i knew who were divorced with kids kept theirs to have the same last name as their kids. Also, i had just finished grad school about a year before, and had several professional licenses and a relatively new job (in a large clinic, establishing a practice where people had to ask for appointments or schedule them by listing or being given my name.) It seemed like a bad time for a name change. Here i am 10 years later with the same name. I have no attachment to it, though i guess it is still nice to have the same last name as the kids. They wouldn't mind, as we have been divorced for most of their lives at this point. I have no feelings for my ex and have firm boundaries with him. Maybe one of these days i will change it, but it will be a huge pain. 

BM took DH's last name when they married at 18. She kept his last name after they got divorced at 25. She got married again at 26 and took DH#2's last name. She kept that name until they got divorced when she was 34ish.

THEN, she changed her name BACK to my DH's last name because, according to her, the school was confusing the boys' last name with her now-XH#2 and the boys were "upset" they didn't share a last name.

So, she went back to DH's last name. She even made a Facebook post about changing back. And my MIL welcomed her back to the family name, which is hilarious because MIL and FIL are divorced and it's FIL's family name.

LESS THAN TWO YEARS LATER, she married DH#3 after dating for 4-6 weeks. Now she has her new DH's last name. The boys must have gotten over the school screwing up their names and not sharing a name with BM (hint: the boys gave zero fracks about BM's last name).

For me, after my first marriage ended, I kept my XH's last name. I was young (25), and everyone professionally knew me by his names. I was embarrassed to be divorced so young, and was afraid people who didn't know that I got divorced would assume I got married. I didn't want to deal with "congrats!" followed by me explaining that I was, in fact, getting divorved.

I did change my name to DH's when we got married. Looking back, and all the hassle it is to show the "chain of name change", I wish I would have just gone back to my maiden name and called it a day.

My DHs XW has remarried AND kept my DHs last name.   She never changed it after marriage.   Yup!   
When I divorced my XH I kept my XHs last name only for the sake of my child.   But had no issues changing it after marrying DH.   But all my BSs friends still call me MRS XH or Mom XH..because of my BSs last name.   My DH doesn't mind and gets why they would do that.   
I think BM keeps DHs last name for the kids and to stick it to me.   YSS once said that BM was going to change her name but it was really expensive.   Insert another eye roll.   It doesn't cost anything except maybe a few bucks here to get your name changed after Marriage.   I'm sure BMs DH feels like a loser because BM didn't change her name.

This isn't an issue where I live. Your maiden name is your only "legal" name. Yes, most women go by  Mrs husband's name. When you get divorced, if you want to continue to use your ex's name, for whatever the reason, you have to ask his permission... 

BM didn't change her name when she was married to my DH and she didn't change her name when she remarried her current husband. I have to give her one thing she's definitely looking ahead!

While she was engaged to her current husband, right after my husband and I got married and I took his name she would send messages to SD about how she was thinking about changing her name when she got married. Obviously DH could have cared less look what she was going to do in her current marriage I think she was trying to make him think the current husband was more legitimate than their marriage was. he also suggested that SD would be changing her name to the new husband's as well. I think she said that because her previous comments weren't getting any reaction. My husband definitely reacted to that one and he said that the only way BM was getting permission to change ssds name would be if he signed away his parental rights and stop paying child support. BM change the tune immediately after that and neither She nor SSD change their name.

There was no way I was chanign my name when I married DH.  It is too much paperwork and effort plus the expense of getting a new passport.

I have no idea what BM uses as her last name as I have only had three very brief interactions with her in nearly 20 years and what her name was didn't feature.

When DH and BM divorced, BM made a big production about how she was going to change her name back to her maiden name. It even lists in their divorce decree that she will be known going forward by her maiden name. However, she never changed it. She got "married", but not legally to someone else and uses his name on social media, but still uses DH's name for everything else. She's told DH that she can't change it because she's known as his name professionally and people wouldn't know who she was if she changed it. I think she can't change it because it would be a hassle to change to her new "husband's" name without a marriage certificate. She could change it, but it would likely involve court documents. It drives DH crazy. 

I didn't change my name when DH and I married. That also drives him crazy, but I point out to him that names shouldn't mean much, because BM still has his. My view is, why should I change my identity because I got married...and DH's family has been incredibly unwelcoming, so why would I want ot further align myself with them. I've always liked my last name. It's unique and pairs well with my first name, so I'm not in any rush to change it. 

Got remarried using your ex last name ? 

I'll add this on there too. BM and I have the same name. Yes I know... gross haha. We spell it differently though. I've thought about what if SO and I get married her and I would have the same name. (Vomit) Thankfully she took new husbands last name from what I can tell. (Not sure about legally) My guess though she changed it legally based on how BMs husband is. 

I can't keep up with BM's last names.

Many Judges will not allow for a name change that makes the BM's name different than a child's.  Once BM remarries, it is more acceptable for a Judge to allow it.

Interestingly, during my divorce the Judge grilled XW on whether or not she was pregnant during the portion of the divorce hearing where he was responding to her request to return to her maiden name.   She got lippy with him and he threatened to require her to have an amnio to verify both that she was not pregnant and to confirm that if she was it was mine.  She calmed down when his honor climbed her ass for being lippy.  

As it turned out... she was pregnant.  Just not by me.  We had not been intimate in a year by the time the divorce was final.

When I divorced psycho exh, I gladly took back my maiden name. I took DH's last name when we married. 

BioHo was an unwed mother when she had SD27. 'Ho went by her maiden name, but gave SD27 the bio dad's last name.

Along came my DH, who foolishly married 'Ho. SD24, SS21, and SS17 have DH's last name, even though SS17 is not his bio.

Spawn13 has the last name of HER bio dad, who dropped 'Ho like a hot rock because she was (surprise!) cheating on him.

After many, MANY fuddies, 'Ho finally managed to reel in Mr. Mr. Pinhead. When they married, she hyphenated DH's last name with Mr. Pinhead's because 3 of her 5 kids have DH's last name. 

'Ho's mother also lives there. With so many last names in the freaking household, it sounds like an apartment complex listing!

I kept my exDH last name, no I don't have hots for him, just didn't feel like bothering. And we do have a kid together. I got married second time few  years back and changed my name to my second DH name. Honestly I dislike my maiden name and didn't mind changing it both times  

my DD is a widow (young). She never changed her last name and kept madden name. She is getting married second time and will continue keeping maiden name. She doesn't believe in changing it 

My DHs ex hated him so much that she didn't want his name but they were married almost 30 years so going to maiden name made no sense to her so she changed it to random name she made up like Ms. jumpoverthefense or something 

After we divorced because I wanted my name to match my sons and back then divorce was not as common as it is now and I wanted people to know I had been married.  I was a good catholic girl back in those days and it seemed important. 

Then honestly I kept my ex's name so long before I married my DH that it really felt like my name and had nothing to do with my ex. If something had ever happened between me and my DH I would have gone back to my first husbands last name again.  It matches my child and feels more like my name than my maiden name.  

Crazy BM in our situation got married and changed her name to her new DH's last name on social media etc .. but we found out she hyphenated her name and really most only use the first of the hyphenated name when referring to someone.  It annoys my husband and he wants her to change it.  It did annoy me at first but she kept it for different reasons than I did. But I soon realized she can have the name I have the man!