> Skype, and arguably facebook, control their markets.

Facebook users have invested a lot of time forming their social connections, posting photos, tagging photos, etc. On the other hand, most Skype users just have a buddy list and people that they talk to. There is no 'anchor' keeping them from jumping ship to another client/protocol (or even running that client/protocol in parallel with Skype). Skype is not the anti-trust that you are looking for.