I like to message people to ask if they're okay, as soon as possible too so I don't see if there is any activity to indicate they're okay. Sometimes people forget and that's fine. Sometimes people do things with the best of intentions. Some people just suck at life by accident. Others however suck at life by choice and couldn't care less about things. I get that people have lives to live. True friends will contact you out of the blue. I had one call me yesterday and we talked very briefly, but we talked. I have other friends I can go years without seeing and we're still on good terms. I get caught up in life too sometimes and forget thought I do try not to. I also don't want to feel like I'm annoying people by contacting them too much.
I figure in any relationship one person always cares more than the other. Many are so easily broken down too, usually over irreconcilable disagreements, simple misunderstandings, or just plain "I don't like you anymore" with no real reason other than they don't like you anymore.
Some people are just friends of convenience too, like you're stuck together in school/work and so you make an effort to get along. Once you're no longer have to deal with each other though the other person falls off the map and maybe you get an e-mail or failbook request from them if you're lucky.
I'm good friends with a nice married couple and used to hang out all the time, go on trips to places like D.C. and the U.P., and they'd even come over and watch T.V. with me and my family. Then they got jobs and I hardly ever see them these days, although I still text them stupid shit all the time. Life just gets in the way and I could call them up to hang out, but again I don't want to annoy them (not that they'd probably care.)
Another good friend got a girlfriend and hardly goes out now. Another got married and had a kid, they go out even less (maybe I'll see them on New Years Eve, maybe.) It's feels like they died sometimes.
I suppose in the end all anyone can do is try to find people they care about, and true friends will stick by or at least keep in contact. Anyone else isn't worth the time or effort if they aren't willing to put any in themselves.
Sorry if I've rambled, I tend to dwell on things.