An amusing addition to this discussion - my Ds call their maternal grandparents by their first names, which was the grandparents’ wish from the start. 
First names. We’re all adults. Maybe when they were kids the Mr/Ms lastname made sense, but with adults the same age as my colleagues and friends, it would just be weird.
First names, definitely. We’ve always introduced ourselves to our kids’ friends that way. The only odd exception was during the years when I had a GS troop, and because of my co-leader’s preference, the girls called us Ms. Co-leader and Mrs. Welsch (as it were).
Our kids call us mom and dad, although our daughter figured out many years ago that hollering my first name in a crowd of other parents and children was more effective if she really needed me.
I don’t think he’s called my husband or me by name at all. Ever. So I guess I don’t know. Of course, they’ve only been going out for three years or so. 
I’ve said this before here, but my in laws NEVER addressed what they would like to be called or invited me to call them anything. And I felt shy about asking. So for the years I was married before they died I…just didn’t address them in any way - and I always HATED that - it felt awkward and I think I always felt a bit like an outsider.
So, my suggestion - lightly address “name calling” with your kids and/or their S.O.'s !!!
We’re on a first name basis. They call me Classic.
First name - for both SIL and DS GF.
As my kids traveled, during university and after, they would jokingly call my Mom in the language of a country they visited. Then I had grandbabies and that became my grandma name since my mother is called grandma and is a frequent part of the kids everyday life. My kids have riffed on that other language Mom name to make it their own so, now even the 2 year old calls me that grandma name, as do the son in laws. I cannot recall if the SIL called me something else before the wee ones, but as I read the posts I am realizing how incredibly comfortable this third name is, not my first name, not Mom, but a name just for our gang.
I called my MIL and stepFIL by their first names, I knew them through work before I knew DH, and bioDad, Dad. My DH & my siblings spouses called my parents by their first names.
Grandkids are being taught to call non-family adults Miss FirstName and Mr. FirstName where appropriate and I wonder how that will translate later into being picked up as a style
My girls still call me Mommy, whether in private or public. Even when they tex/email me, they’ll write Mommy. I never thought much about it until someone pointed it out to me.
People I meet at work, young or old, they all call me by my last name. Outside of work, I don’t really interact that much with 20-30 something other than my kids friends or my friends’ kids. I would never correct anyone, but it gets my attention when they call me by my first name.
When I meet my friend’s parents, I would usually call them Mr & Mrs unless they tell me to call them by their first name.
@oldfort , I think that is so sweet that your daughters call you that. If I had daughters, I would love it. I also still call the generation above me Mr./Mrs. , even my best friends parents. You know, I actually like it! It makes me feel youthful to say. I’m taken aback when a friends child calls me by my guest name. I don’t like it.
H and I are Mr. and Mrs. Charms to all of our kids’ friends, including SOs. Our children are 19 and 21 and we have known many of their friends for years; that’s what they called us when they were little kids (or middle or high schoolers) and they still do. To my knowledge, my kids address their friends’ parents the same way.
Their college friends also use Mr. & Mrs. even though some of them were 20 or 21 when we first met. Honestly, I prefer the formality and wouldn’t suggest that they call me by my first name. Mr. Charms is even stuffier than I am, so there’s no way he’d suggest it.
When they get engaged, we’ll have the “Call me Lucky or call me Mom, whatever you’d prefer” conversation.
I do not care for it when people introduce me to their children as “Miss Lucky.” Our last name is simple to pronounce so there’s no reason not to be addressed as Mrs. Charms by even a young child. Apparently this is quite common and appropriate in some regions (my niece from NC does it), but we are in the mid-Atlantic and it grates on me every time.
My future SIL and D2’s boyfriend both call us by our first name. D2 asked if her boyfriend could call me mom. I told her no. If they were engaged it would be ok, but not when they are just dating. She said he called all his friends’ Moms mom because he didn’t have a mom growing up. But I feel too uncomfortable with that.
@oldfort, my 26 year old daughter calls me MaMa. I never pursued that name, always thought I would evolve into Mom eventually. But, when she calls me and I hear her sweet voice say “Hi MaMa.” It just tugs at my heart. My 21 year old calls me mom or mommy.
Typing too fast at work…I meant to say they call me by my first name, but you guys knew what I meant.
What is this mythical “significant other” of which you speak? Three kids all flying solo. Our exchange students the one year called us by first names, though neighborhood kids addressed us by Mr. and Mrs.
My wife and I call both sets of parents Mom and Dad. We are a personable group and that is the most authentic to how we feel. I am a fan of connections being comfortable for both parties.
This is very southern. Many kids called me Miss Firstname when we moved there.
I remember visiting a friend/former boss who had young children. She asked me what I wanted them to call me, like Miss Mary, Aunt Mary, Miss Lastname? I went with just first name but we discussed not only what I was comfortable with but what she was comfortable with her kids calling an adult (I was probably about 30 at the time).
I was Miss First Name when we lived in NC. I actually liked it. Made me feel younger. When we moved to the midwest, I introduced myself that way to friends’ children only to have them correct their children and say, “no, it’s Mrs. Last Name”. Mrs. Last Name was my mil.
First name.
I always introduce myself to kids’ friends as firstname, just to set expectations. My parents never called their in-laws anything because they never had that conversations, and I don’t ever want to be in that situation.
My mom is in the south and gets called Miss Firstname by people who know her well but are my age or younger. It’s charming to me because I grew up with it, but probably hard to adjust to otherwise.
I always introduce myself by my first name, but I think it’s easier for some people to stick with Mrs…then they don’t have to remember my first name, only my kids’ last name. When I was involved in school activities, I recall all of the kids addressing adults as Mr and Mrs. The SOs usually do fine with first names, once they’ve been around for a little while. My husband and I don’t have the same last name, but we usually don’t correct people just trying to be polite.
Someone posted about using foreign words for mother. I realize I used E-Mah , Hebrew for mother, fairly often. So, you could be EMah Oldfort or Emah Lucky, or EMah Bet. I could,possibly introduce the equivalent of Auntie Bookworm for the futures.
In the meantime, I am just plain first name