I went through a lot of the same in the early 20's part of my life. Here are a few concepts to consider towards the dynamics of what's going on with your friends.
Your friends' interests may be compelling them moreso than ever before, and with more conviction. They are no longer on the terms of their parents or bound by the social order at school. It is now your schedule, your time, and people begin to gain confidence to pursue interests and relationships that can have very different dynamics. As such, immersing themselves in new relationships requires effort and a rearrangement of priorities and focus. In a way they are developing new personality aspects, so while they are gaining confidence in their new directions, they still have insecurities. That might make it hard for them to jump back and forth (the whole non-acknowledging you thing). They may just not know how to seamlessly relate to everyone, or they might feel insecurities around their new friends.
It can be a really tough process to handle, but I can only suggest that you keep finding new interests, and really try to get involved in them. Whatever it is you do, try not to dwell on what others might be thinking, or overanalyze the group dynamics. That just ends up being a repetitive cycle, like listening to the same song over and over again, hoping that you can recapture some good feelings or glean a new insight. Ultimately it may be a way of passing your time, but will not really be too rewarding.
There's also nothing wrong with calling friends occasionally, but you probably have to let them fly on their own for a bit now. Don't hold them accountable to loose commitments like "I'll give you a call sometime." Groups form and dissolve constantly in the social world - there is a common energy that brings you together, and then it subsides, and you find new interests. It can be awkward though, the compelling feeling that you should still be hanging out with them. When you get together or see each other in passing, it's like you're in the old house, but everyone has moved out.
Probably a good move for the long term would to be to leave good impressions. If they feel like you are going to be holding them to old ways, or trying to hold them back from taking new directions, they might avoid you more. The smart move is to let them go, keep on keeping on, and at some point maybe some of them will come around. In time they may regard you as a great friend, or maybe as someone they want to hang out with at times, or possibly as just a passing acquaintance where you make some small talk at best.
Make sure you don't project anger at them, if at all possible. While it seems logical that you are just blowing off some steam, it may not have the same effect on them. If you are respectful of their new ventures, it will help them have a better appraisal of your relationship when they consider it, and give them a lasting feeling that they trust you.