Woah there, wait just one minute. Facebook had 5.5 million active users after two years [1]. Foursquare has 7 million registered accounts [2]. I shouldn't have to explain how different those two metrics are.
ah missed the "active" part ;) I quoted that from memory on my phone--not an apples to apples comparison but makes me wonder. I'm interested in what portion of 4sq's users are 'active', so I'll see if I can do some digging and find out.
[1] http://www.facebook.com/press/info.php?timeline (active user defined as having visited in the last 30 days)
[2] http://techcrunch.com/2011/02/21/foursquare-closing-in-on-7-...