The day Facebook launched 'Places', it's geolocation service, Foursquare reported experiencing record signups. Places works on the same principle as Foursquare; people check into places and frequent check-ins earn rewards. Notion was that Places would completely outdo Foursquare but according to a tweet by Foursquare CEO, Dennis Crowley, Foursquare had its 'biggest day ever' in terms of signups the day Places was launched.
Foursquare is approaching 3 million users but that in no way compares to Facebook's 50 million. While the one day may have been a record sign up day, only time will tell if Foursquare will REALLY be able to fend off Facebook.