Good question. The number of users required to start delivering value is just one! After all, if you use a bookmarking site like http://www.blinklist.com you can immediately start it to store and retrieve your favorites from any computer. In addition, finding links becomes much faster.

That also means you only need to use one bookmarking service. True, for the social part there is much too be gained from having more users. Again, however, all you really need to do is get people like you on the same service. That is much harder to do and the only site so far with lots of momentum is del.icio.us.

However, it is still very early in the game.

Mike