Facebook announcement of Places service is an important event in the Platform race. Facebook could have partnered with Foursquare or some other geospatial player(s) . Instead, Facebook is going for leverage strategy, aiming to build and control its own "wide platform".
Possible strategic response for Foursquare, threatened by Facebook ambitions and lock of the "wider platform", is to integrate with a new, "open" social platform such as Diaspora. It would be a mistake for Foursquare to try to build its own social platform, unless they make it "open" and convince other major players like Zynga to integrate with it.