I think the people that we classify as "they", and as "BPD" on this site, have feelings like every other human being. I strongly believe that even the worst people in the world have feelings somewhere. So if we meant anything to them, yes, i believe they think about us too and they eventually miss us too.
Reading the posts and information about borderline, it often makes me feel bad about how we talk about those people, that we suspect to fit the criteria. We make them sound like such monsters with no heart and soul. Isn't that the easy way out?
I honestly believe my ex loved, and still loves me. It's a very unhealthy love (from both sides), but it's still what she would describe as "love" if asked. When i didn't do something wrong, sometimes for a week or a little more, we had really good times, like a healthy relationship. I believe we both didn't play just a show when we felt a deep connection to each other, when we both were amazed and so happy the way we felt in each others arms, the way we could relax or laugh together, the few trips we made together... . call it two dysfunctional people and an unhealthy love, but i believe the feelings were always very mutual from both sides, at least they were (and still are) from me and her.
It sttill doesn't make the relationship work however... . it's still dysfunctional.
I did my share of bad things, but never out of the blue i started criticizing her. This is what really hurt me, and i know it will never stop. Even if in those moments she fears i don't love her, and feels abandoned, and i should just take her in my arms and tell her "don't be scared, i love you". I know she will never be able to just say how she feels in these moments, and i know i myself will never react in a calm way to her belitteling words in these moments. People don't change very much, and usually not very fast. So it's really a doomed situation with no good outlook to ever become a healthy relationship.
What i'm trying to say i guess... .
I believe if the relationship you had with them felt deep in some way, connected... . i don't know how to say it. If you always trusted in each other that you both really love each other. I always had this with her... . this is why it's so hard. I know she still loves me too, misses me too. I think they all do if the relationship meant something to them.
This is why i refuse to label her as "BPD" (even if i told her this in an angry email towards the end). Yes she has traits that could be labeled as what we call BPD, but in the end it's just a person with quite a few problems that I can NOT solve. Like she can't solve mine FOR ME.

Sorry for venting / ranting here... .
I'm simply trying to say, yes i believe they miss us too (as above, it depends on the individuals, sometimes the other person really doesn't care, happend to me as well). This makes it easier in some way, and harder in another... . Sometimes i wish she wouldn't care, because it's so much harder to say stop and walk away... . i know her love is not healthy, but it's still love for her... . :'( :'( :'(
Sorry, i guess you got me somehow with your post... . this is hard... . :'(
I believe they miss us too.