The web is _very_ churny; waves of people move from service to service over 5-10 year adoption cycles. Eventually people will declare twitter "uncool" (perhaps as a result of abuse problems?) and move onto the next thing.
The modern web has been very churny, but I think we have been witnessing the equivalent of the Cambrian Explosion [1]. Eventually both tech and behavior will settle down. Some evidence for that was the rapid churn cycles in social tools from 2002-2008 or so. But Facebook, Twitter, and LinkedIn could well be where the musical-chairs game stops.
