Mashable has a post about how Facebook will look in 40 years, "once the younger generation is playing Bingo and having hip operations."
This got me thinking about if/how we actually will use Facebook in the future. While I can see it becoming irrelevant for the people who don't care about being connected with their friends, I believe it will still be relevant for many of us. Once you're used to being able to check spur-of-the-moment on what your friends are doing, it's hard to quit. Most of us desire connection in some way/shape/form, and Facebook gives us this--it may not be as deep a connection as it would be if you call someone on the phone or talk to them in person, but this doesn't make it any less real of a feeling of connection. The beauty of Facebook is that it allows us to at least feel like we know what's going on in people's lives (thanks, mini-feed), even if it is just a small slice of what people reveal. And we don't have to spend a lot of time doing this, though I'm sure some people will continue their stalker-behavior for life and continue spending hours each week using the site. In 40 years, the only thing that will change is that there will be more old stalkers...great.
While we might not be adding as many new friends in the future as we are now, I think we'll still be using Facebook, just as 20-30 year olds today still play video games, even though many people thought they would grow out of them.