Facebook is no stranger to encouraging or suggesting user actions. The site regularly suggests potential friends as well as encourages users to connect with friends who haven't been active on the site lately.
But it appears this may be the first time Facebook has actually used peer pressure to encourage action. This new message, apparant to visitors who sign in, encourages users to scan their Skype address book for friends that they might not have connected with on Facebook.
Previously, Facebook has used game dynamics (i.e. a leaderboard) to encourage users to invite more friends to the network.