I bet it was more viral than anything else.
So, I tried out foursquare.com on my iPhone tonight...pretty cool, but looking at their traffic history, it does bring up some interesting points when looking at other sites.
Site Profile for foursquare.com (rank #1,416) | Compete
What dynamic event happened between August and September 2009 to make their "compete" view of uniques jump from 22k to 215 K...
also what did they do from Jan to Feb 2010 to go from: 592K to 1,200,000 ?
Was it simply VC money? Linkbuilding and marketing? Viral spreading of the app? Or all 3?
may be a feature in techcrunch or simmilar big blogging network
Foursquare Shows The Business Potential Of Location-Based Services
I think it is viral, well that's the magic of viral videos