Thanks for sharing, I truly believe a "fast mover advantage" is super important in Startups, especially when a window of opportunity becomes obvious, it's all but a race of speed to market by a multiplitude of startups.
Having said that, I wouldn't try to steal users from Facebook or Google in the same market today because of the "network advantage" they have created over the years - it's where your friends use, and that makes the website sticky. Maybe there are ways to do it, but I think it's more efficient to try create something completely new.
Perhaps Foursqaure or Gowalla isn't that sticky yet?