Except there is no metadata to analyze. That is the point. They are very open about the fact that the only data they have is when your account was created and (i think) your last login time.

The phone number was a stroke of genius because your phone contacts is your social network. Really easy bootstrapping off the existing network.

There are other options that don’t require a phone number, but predictably they are less popular. Try Session, Status, Matrix, XMPP, etc.