Send some emails, questiony FB wall posts, texts, whatever and see if anything comes back. Maybe they're neglecting you but they don't realize they're doing it to the point of making you feel bad. If you don't get any response, i might let em go. Unless they are a BFF and worth fighting for.
Other possibilities: my partner and I had a friend couple we used to hang out with a lot. They went through various rough patches in their relationship and we got the feeling that being around us was making them feel lousy (we have a really good relationship, not trying to brag but we get along really well). They called us less and less and even though we all like each other, we decided we would amenably have a friend break-up with them because we thought we were making them feel bad.
Yet another possibility: Some friends of friends of mine have dropped out of contact because they are jealous. Like, Friend 1 has a nice house, a nice kid, a nice husband, a decent job, and Friend 2 has a jerky husband, naughty troubled kids, a messed-up house, a lousy job, and instead of being happy for a friend or even asking for advice or whatever, Friend 2 is too resentful and jealous to continue the friendship. Even if you aren't a braggy type person (which I bet you are not), if things seem to be generally going your way it could be that your friend isn't able to deal with it.