Most of the ones I've had either moved away, my family moved us away from them (back before any of us had our own phones), they threw me under the bus for their own gain, or just moved ahead with their own lives in becoming adults.
Supposedly, I still have two left, but it's increasingly feeling like zero. One rarely has time to hang out, never initiates communication, and doesn't want to do anything to maintain the bond we've had since I was in late elementary (so almost eighteen years now). The other disappeared for like a month recently, rarely even talks to me now after coming back, half of what little discussion we've had since are just me being asked questions that feel more like insults about my character, won't explain what happened, and won't even acknowledge my wanting to catch up on what's been going on in both our lives. I really don't know what to make of it.
No other vectors for me to make friends anymore, and even if I had one, there's too much wrong with me physically and mentally to trust anyone to stick around once they know about it. I've felt like those two up there have (had?) only been doing so because they knew me before things got to this point.
Chase? No, that can come off as being invasive and prying. Show interest in their lives, and if they live nearby, do things together occasionally? Yes. Granted, a lack of communication doesn't necessarily mean a lack of interest, since some people do simply get busy with their jobs or starting families of their own, but it still creates a decaying state leading into complete indifference. My parents have said before that it's completely natural, and to be expected, because that's how adult life is.
>Maybe I should be always the one who go after them when we just don't see each other for other reason (like class?
That could help, but you have figure out if they're the sort to be receptive or not. Actually kept one of the above from anheroing some years ago in doing that, after not having spoken since the end of high school. Still, a friendship (or any relationship, really), should be a two way street. One party shouldn't be the only one trying to keep a connection going.