Facebook has put a lot of effort into driving more 'friendships' through their Friend Finder tool. And Twitter has had great success with their new, excellent "Who to Follow" tool. Facebook already integrates with GMail, AOL Instant Messenger, ICQ Chat, Windows Live Messenger, Yahoo! Messenger... and now Skype. And for Skype, Facebook has specifically promoted the integration very heavily.
These examples are from the last couple weeks (sorry for the delayed post), but notice how prominent the units are. They are placed on the homepage and beneath the header / search bar.
As Facebook attempts to drive deeper usage and more friend connections - they must be running out of partners with any reasonably close scale. Skype makes sense of course because: they are one of the few brands / services to big larger (by number of users) and to be a mix of US and international (big focus for Facebook).