Yeah, real college has this effect on friendships, it seems.
I didn't realize how hard it is to keep up on HS friendships until
I was the one on the 'keeping up' end; previous to my graduation and leaving my hometown, I had older friends that took off to university and I
never, ever heard from them. Or they were really flaky about making plans, and I would only see them maybe at Christmas.
Then I went off to 'real college' (I say that because I went to community college in my hometown for two years -- which makes it easier to keep up with old HS friends). I feel terrible, but I didn't talk to most everybody that I was good friends with. One guy, an oooold friend (we're talking since 6th grade here) I didn't have contact with for over half a year. At the moment, I don't really talk to any of my old high school-only friends; only the friends that have weathered with me through school changes and house moves -- the ones from elementary school -- are the ones that have stuck with me. And they are 3 in number.
But it sounds like your friends have been the ones that didn't hold up their end of the friendship? Sometimes I blame myself for not communicating with people and letting ties break down over time, but then I remember that it takes two to make a friendship

. If they don't get in contact with you, it's just as much their fault as it is yours. Of course, I've had "friends" that kind of float from one person to the next -- one of them I was quite close to, but as soon as he graduated he just disappeared off the face of the planet, even though he only went to the aforementioned community college -_-.
Jam it back in, in the dark.