Conor Friedersdorf asks "Is Twitter too big to succeed?" I'm inclined to say yes. Or if not, it will be.
I don't want to get into too much detail, but the trend in past social networks is that they grow in popularity to a point at which the chaff outweighs the wheat. At that point people find it too hard to keep track of those they actually care about amidst all the updates from friends-of-friends-of-that-guy-you-once-sat-behind-in-History about whether last night's Dancing with the Stars was any good and they abandon the network in favor an upstart with a better signal-to-noise. Maybe Twitter will be different, but lacking evidence to the contrary I'm not sure why we'd assume it would be.
If you don't believe me I'm sure one of Jenn Golbeck's papers will back me up, I just don't have time to figure out which one.