One has to ask what Skype and Microsoft hope to get by allowing Facebook to offer a video service to its 750 million users worldwide. Some might say exposure, but apart from a non-clickable logo that appears while the video session is initiated, there's not much sign of a balanced deal.
The VoIP service welcomed the social network to its desktop environment back in June 2011 with a "Deeper Facebook Integration" built into the Skype 5.5 Beta, which allowed users to instant message their Facebook friends directly from Skype simply by clicking a tab.
Skype stopped short of allowing Facebook to offer an import functionality which would allow users to bring their contact book to their Facebook friends, and it hasn't yet brought voice calling to the social network. This means that Facebook and Skype users won't be able to communicate with each other by voice for now.
We don't know whether there has been a financial arrangement between the two parties (who share Microsoft as a common investor) but this is highly likely given that Skype has had to make significant investments to build up capacity ahead of yesterday's announcement.
As for Facebook, it receives the kind of exposure on the desktop (and the 600 million or so Skype users) that it could only dream of and the Skype partnership means that it doesn't need to offer a standalone Facebook desktop client. It will be interesting to see whether Skype starts to account for Facebook Skype video calls next time with their next update.