Skype for Windows offers deeper Facebook integration

Skype have released version 5.5 for Windows which now allows checking the availability of Facebook friends without leaving the Skype client.

There are also improvements to the group chat functionality and the video quality.

Though Skype currently dominate this space, Vonage are releasing mobile clients that will allow VoIP calls, there is already a Vonage client for iPhone which integrates with Facebook chat (but doesn't allow access to other Vonage users).