I have long believed that Facebook/MySpace is like a TV show. Once the "content" becomes less interesting, people will slowly tune out. I am personally seeing fewer and less interesting updates from my friends.

In fact, the general theory of mine is that any business whose key metric is "time spent per user", is basically a time sink AND in the media business and needs to keep up with new ways to entertain people. Those businesses have shorter shelf lives.

On the other hand, Google/Microsoft are in the productivity/time saving business and they have more durable advantages since they don't have to keep coming up ways to make people stick around. People stick around and use their service, because people save time.

Watch out for Twitter to do to Facebook what, Facebook did to Myspace and what Myspace did to Friendster.

Twitter on the other hand may not exactly be like a TV show as much as TV channel with each mega Twitter star (e.g. Charlie Sheen) a TV show within that channel.