Kevin Rose asks the interesting question of why we move to one service to another every few years. I think it’s partly because of the new shiny thing but mostly because current social services do not optimize for relationship changes.
The only way to keep Facebook and LinkedIn up to date is to add new connections, delete old ones and create groups. On the latter two, it can be potentially awkward and time consuming.
What if these services created algorithms to optimize the experience over time? For instance, what if Facebook took into account age, life stage and changes in the social graph? What if LinkedIn optimized experience based on new roles and companies you joined?
Ideally, for existing services to flourish, they should become more intimately tied to our real lives via predictive algorithms. Otherwise, as Kevin points out, the risk is just starting over with a new service.