"facebook 'cause everyone is on facebook."

It's the same effect.

There are only two things that are going to fix it - either something amazingly better comes along (facebook vs myspace) or the incumbent royally screws up (reddit vs digg).

To be fair, it's not entirely the same sort of thing. Git is an open protocol. Github just happens to be one of many sites hosting git repositories. Moving a repository to another host is trivial; you already have all the data in your local tree.

Facebook, on the other hand, is one of a kind. I absolutely must have an account there if I want to do anything Facebook. And good luck getting your data out or interoperating with other social networks.

This isn't about the technology, it's about the people using it and how they choose to interact with each other.