Another monopoly in the making: "github cuz everyone is on github".
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).
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.