So what do you want to use to keep in touch? The telephone? I don't think people would appreciate it if you kept calling them every few days asking, "What are you doing now?".
We don't need to know this. We (this social media culture/generation) spend more time finding out what other people are doing than actually doing anything. It doesn't matter what your friends or your brother or your aunt did today or last night or last month even.
As a casual user who visits Facebook a couple times a week, I find it to be tremendously valuable. Contrast with email for a moment. I somehow made it into my inlaws' family list. This is a group of perhaps 100 families who all manage their own copy of the list via Reply All. Many of them post multi-page updates of their domestic tasks several times a week. I can never be removed from or block this list because I may receive legitimate email from any of these people. I may occasionally actually be interested in what they are doing, but as it is the problem is worse than spam by a long shot. Now that some of them are getting on Facebook it is a huge improvement. I can block the truly annoying people and still keep ambient awareness of what's going on on my own schedule.
I think Facebook is a tremendously useful tool, which is often obscured inside the echo-chamber by the feeding frenzies around virality and engagement. In other words, I believe Facebook is sustainable.
Every time a user changes his status message it gets logged as if it were a tweet. It's more like facebook than twitter, though, in the sense that only mutual "friendships" would be possible (could be made configurable: "allow anyone to see my status updates, but not my online status, and only allow mutual friends to chat/talk to me").
It would be a pretty good signal to noise ratio for most people.
Another idea that I have for the future of social networking is that it will open up again, or become less private. The problem with facebook is that it has become a glorified rolodex where you can list and keep track of everyone you know, but you don't find or meet anyone new, anymore, because everyone has private profiles. Thus, the only people you friend are old classmates, relatives, or people you meet in the "real-world". It used to be the opposite, I used to friend new people all the time by searching networks with like interests or by stumbling onto people through friends of friends. People will either need to loosen up, or use virtual aliases so that people can actually network and find other like-minded friends on "social" networks, again.