Technology is only one part of a solution. And in this case arguably a small one. The reason people use Skype is because all other people are on there. It's also synonymous with voice and video calling. Owning a "position" is an other important value.

Facebook seems like the only company that can bring a real network of users to something like this. Google has GMail, but they haven't been able to translate it into any type of social experience at scale.

Interestingly, Facebook is another player that also uses XMPP for messaging. If they really wanted to get into the VOIP game it would be relatively trivial for Facebook to roll a client, implement federation and allow the network effect to run wild.

Agree Facebook is the player that could be a serious competitor to Skype.

But their strategy would indicate that they wouldn't implement federation. They would create their own closed system. Facebook is all about growing the value of their social graph, but with very limited interconnection or federation.