Sometimes I feel Facebook get weaponized just as much as children do in divorce and, in particular, step situations.  Just like you can't force someone to love you, you also cannot force someone to accept your Facebook friend request.  If I didn't know any better, I swear the serenity prayer was written specifically for step-parents: "God, grant me the serenity to accept the things I cannot change, Courage to change the things I can, And wisdom to know the difference."  A couple of things come up: How does her father feel about this?  I'm guessing somehow this 13 YO is getting the idea that you and her father are not connected or on the same level as other family members or friends.  It might be something as "simple" as she wants to stick it to you, to something much more intense as PAS'ing from mom or other so-called family members.  That might be something to give more thought to--the overall cause vs. just focusing on Facebook.  But, keep in mind you may not be able to do much with that cause either.  This is something your DH will have to largely address, being dad, a blood relative, and legally responsible for the child.

However, I certainly get the feeling that a lot of SMs have here, and that is you have done a lot for these children, been very involved in their lives, have tried to contribute positively to their being and so on, and after many years are now feeling or continue to feel excluded and invisible.  You'll get a lot of validation and support and ideas for that here on Steptalk.

And, I do think your friend is correct--teenagers don't think the same way we do.  However, I'd also add that I don't think of that as any kind of excuse for much of anything, especially rude behavior.  If everyone keeps excusing bad behavior from young'ns as "just the way teenagers are," then I'd say that is probably why they are the way they are--everyone is too quick to write off their bad behavior to teenage hormones.  Also, what some of us oldsters used to think of the teenage years--ending at age 18--has now been extended by our society to near age 30.  There is a adultolescent period now that goes on from about age 18 or so 'til age 30, where you never know what you're going to get.  Doesn't make it easy on anyone, and in particular on step-parents.

But, overall I'd say your DH, in particular, needs to address any of these feelings with his own daughter.  Makes it much easier on you and SD if dad owns what is HIS divorce and the behaviors of HIS child.  It is much more-so his job to promote his wife as his wife or his SO as his SO than it is yours too.

The first thing that you need to accept is that we cannot change another person's behavior.. only our reaction to that behavior.  Secondly, while it may seem very personal, it is much more likely that what is going on is actually not personal and about you at all.

Given the ages of the kids when you got together with their father.. you may need to consider what the split from his EX was like.  I am assuming/hoping that you did not enter into a relationship until well after their relationship was over and he was already divorced.  Unfortunately, a bitter EX who feels you were the "other woman" has an awful lot of ammunition in the PAS'ing of their kids.  Even if you weren't the "other woman"... if your DH had left his EX due to cheating etc.. she may still be attaching those feelings towards him getting together with you so soon after their breakup.  And.. the older girl who may still remember her parents together it can be more of an issue.

I also have an older SD that was more of a cold fish to people.  Partly, I think it is just her personality.  In your SD's situation.. it may just be a case of you are two people that don't click.  On the face of it.. that's ok as long as you are able to reside together and be civil.  Her father shouldn't tolerate bad behavior or rudeness.. but he doesn't have to force her to like or love you.

Of course the flip side is that you are likely to not feel inclined to do as much "nice" stuff for someone you don't care for much.. or who doesn't care for you.  And that's ok.. disengaging means you don't put yourself out for someone that isn't inclined to do it for you either.. and it means that you develop a thick skin about the fact that not everyone in the worlld is going to like you.  

The other unavoidable and "non-you" issue is that the girls are both in their early teen years.. terrible time for so many kids and even their own bio parents probably want to dump them in a ditch at times. 

As far as Facebook goes... the girls should be giving their father and mother their passwords.. they don't have to "friend" people they don't want to.. but their parents should have access to their social media and phone

I would also recommend disengaging. Doesn't mean you can't or won't do anything involving skids - just that it is on your terms.

For me it was for self preservation. Nothing that I did mattered. Nothing was ever received gratefully but the opposite. It was met with distain. So I just stopped.

My philosophy is they have two parents. I don't need or want to be another one. In any manner or fashion. And it works!

There is zero arguments over skids because I don't bother. 

There are zero issues with skids because I don't bother. 

There are zero problems with HCBM because I don't bother. 

I am simply the woman dad chose to spend his life with. And our relationship is my number one priority as it is with him. Skids despise that. 

It will only get better if her father puts his foot up her butt and forces her to interface with you when she is in YOUR home.

"Ask your Step Mother.  What she decides goes." Lather, rinse, repeat.

She will learn when she gets zero response to her desires except through you when she is in YOUR home.  Yes it is DH's home as well, but for the purposes of this excercise to improve her behavior... the road to happiness for her goes through  you.

Good luck.

Does not excuse bad behaviors. I got really tired of hearing that, back when Toxic Feral Eldest was living with us. I did what has been suggested - I favored the younger one, took her places, bought her things, make her favorite foods, watch movies with her. We have a good relationship. IT really upset the Eldest who grew to resent me, and still does. She is now 19.5 and living with her mother Toxic Troll. Just recently Toxic Feral sent DH a series of scathing texts, and it broke his heart. Now, during drop offs she scuttles off to her dirty disgusting room.

Facebook. Jeez.  I don't take it that seriously.  When SD15 left our home on very bad terms, I simply removed her as a contact.  I think she was also using some of what I posted as ammunition against DH too (long story).  So she no longer gets access to my life.  DH's sister and someone else asked me if I had removed her as a friend.  "Yes, I did.  I don't trust her to see my private life, and DH's if she uses it against us.  I don't let anyone do that."

Great advice here!  And, remember the first thing a despondent SM has to do is learn not to give a bag of rocks about what anyone else thinks, because no matter what a SM does, there will pretty much be someone there every time claiming it is wrong, especially from non-SMs.  Being a SM is one of those things that everyone else thinks they know more about than actual SMs.

When you think about it, it is like a Medical Assistant thinking they know more about prescring drugs than an actual doctor, or a rookie cop thinking he knows more about police procedures than a tenured detective.  Nuts, but happens all the time with SMs.  Even SS's 17-YO girlfriend of two months will be given credit for knowing more about how to be a SM than a seasoned 10-year SM veteran.  So, let whatever anyone else may think of you or your decisions as a SM go.  Remember, as an actual SM, you are a SM genius compared to most.  Just like a doctor wouldn't let a medical asst. tell him what drugs to prescribe for someone, you should be focusing on going with your gut and experience vs. giving much credence to what others inexperienced in your area are telling you.