Yesterday, Foursquare turned one. You read that right: The mobile social network that made its splash at SXSW 2009 and has tremendous buzz is just 366 days old.

Despite that short amount of time, Foursquare has more than half a million users, 1.4 million venues and 15.5 million checkins, and it’s still growing. Experian Hitwise decided to use this milestone to analyze Foursquare’s growth.

As you might imagine, not only have Foursquare’s mobile apps seen growth, but so has What did strike us as surprising is that the site’s number-one referrer is Facebook, which accounted for a whopping 33% of upstream visits last week. That’s even bigger than Google (22%) and Twitter (8%) combined. It’s yet another indicator of how much traffic the world’s largest social network can drive.

Searches for Foursquare have also been sharply rising, accounting for around 0.00032% of all U.S. searches. Its most recent peak was February 20, about the time Please Rob Me was gaining the press’ attention.

With more than 15.5 million checkins and nearly 300,000 yesterday (many due to SXSW), Foursquare seems to have a bright future. However, competition from Gowalla, Yelp and Facebook in the local space could give the startup a run for its money this year.

Reviews: Facebook, Foursquare, Google, Gowalla, Twitter, Yelp

Tags: foursquare, Mobile 2.0