Other companies often implement Facebook's "Find Friends" feature into their signup processes. Products are more enjoyable to use when people you know are using them too. Twitter used to use Find Friends to suggest people to follow. Path uses it to show which of your friends are already on the service.
But apparently Facebook has had enough. It's utilizing a line in its platform policy that states, "Competing social networks: (a) You may not use Facebook Platform to export user data into a competing social network without our permission.”