A study by PageLever, an analytics tool for Facebook, showed that search engines are major traffic drivers for Facebook pages, making up 34% of all external referrals. Google was the leading search engine sending 27.57% of all external traffic to pages. This study was analyzed 1,000 Facebook pages (each with 10,000 fans.)
Surprisingly, Yahoo! is sending Facebook pages more traffic than Bing (who teamed up with Facebook for a more social experience). Google also sends more than 10x of the traffic than Bing.