I buy into the argument that social networks aren't stable entities. Because the main element is friending somebody, it is incredibly awkward to unfriend somebody, or denying a friend request.

So a boss asks to be your facebook friend, and all of a sudden, you have to be super careful about privacy settings, other friends, talking trash, etc. Now it's become linkedin.

But hey, check out this new social network. Hop on a new network that's growing, and keep up with your friends, who you can convince to also join.

Repeat.