Imo.im supporting Skype and Introducing Voice, Video Capability On All Networks

Interesting that it supports Skype, which is the only protocol in the list that doesn't have third party clients (afaik; if I'm mistaken, please correct me).

That'd mean they either partnered with Skype (which would be a first, so I don't think it's likely), or are using Skype client API and rerouting audio from/to it. That's quite a feat, especially if they can manage that in a scalable (wrt the number of users) manner.