You might be experiencing what I've taken to calling a "Friendocalypse" - I've had 3 group ones since I was about 17 (I'm 26 now)
Though I've tried and at times thought I was succeeding, I've never been able to really have more than 1 or 2 actual friends at a time, though there have been times when I appeared to have a large group of friends when we were in the same extra-curricular or committee group (all ending in friendocalypses) and other times where I've had none.
You're a guy, and guys might work differently, but in my case there was always a kind of self-proclaimed "leader" of the group who decided that she didn't want to hang out with me, and other people followed her because either they agreed or just wanted badly to stay "a part of the group" that they didn't stand up for me (some later apologized and said they were wrong, others never did). Since this seems less sudden, maybe other people are right that interests have just changed.
To keep friends, I know I have to make a sometimes unnatural feeling effort to keep them in my life and remind them that I'm here, and that means calling them up to chat about shared interests (like TV, we chat about the new BSGs, Offices, music, maybe politics if we agree, news events, etc) or inviting them out to dinner, a movie, the park, to sport, just something that forces interaction. They may think you stopped liking them b/c you weren't contacting them even if you didn't "do anything" obviously wrong.
If you try this and they seem cold, turn you down, or it is awkward, forget them. Maybe seek out new groups or activities that interest you and try to bond with them. At the end of the day, if you can just find one person who shares your interests to hang out with, you're better off than a lot of people, NTs and Aspies combined.
You might just need to decide how important social things are for you. I've still never figured out a way to stop having friendocalypses (the last one being in '06) and they never stop hurting, but for me it has been better to have tried and lost than to never have tried at all.
Though I might need to re-think that, since I'm just figuring out I'm an Aspie.
Luckily, even in the loneliest of times, the internet is always here.