There's no doubt that applications such as Facebook, MySpace, LinkedIn and Twitter are popular based on the number of users.  However, that doesn't necessarily mean they're adding a lot of value other than more noise.  After all, illegal drugs, tatoos, fake wrestling are popular too, but look what they do to our society.

 Personally I've always tended to go the opposite direction of the crowd.  For example, I go to the beach in the winter and ski resorts in the summer.  I like it when the crowds are small.

Yes, I am using LinkedIn to some degree, but the long term value is still unproven.