It's funny, I seem to remember reading ages ago that Twitter was going to outdo facebook.
That, I don't think, is likely now. Facebook has (IMO) caught on because it allows people to post photos/videos for family/friends, as well as messages and status updates (or tweets, in twitter parlance). It also allows members to post links, chat (using a sort of web based IM) and play games with their facebook friends.
In short, it performs a function. It is much like the virtual equivalent of meeting friends in the physical world.
Twitter allows the user to fire off short messages at other users, so (in my opinion) is similar to being a speaker at a group meeting addressing an audience or conference. This is not how people communicate.
Twitter will, I believe, ultimately fail, because it provides no other function that allowing you to talk at people. I would even go so far as to say that if only 10% of their members are actually tweeting, they are already failing.
In short, I think that within the year, Twitter will have gone the way that Second Life has (if you recall, Second Life was also going to be the next big comms medium as well), and the way that (IMO at least) blogs have gone. We'll all be thinking "Ahh, Twitter, I remember that".