According to Compete, January marked the month where Facebook passed Google in visits and Yahoo in Uniques... both are major accomplishments. Even if Compete's data is not directly tracked - this is important directionally. In December, Facebook inched above Google in visits (2.71b vs. 2.69b) - but January marks an actual gap: 2.87b to 2.78b. The other impressive feat is that Facebook's growth rate maintains its (almost-linear) strength. Google's visits continues to grow, but it is clearly slower; their uniques have seen growth over the last couple months, but May - November was relatively flat. Meanwhile, Yahoo visits are flat and uniques are declining. It is also worth noting that Facebook is the top referral site of Google... meaning that many of Google's visits lead directly to Facebook visits (as is the in-and-out model of search). Google is the second largest referral site from Facebook - Yahoo is the largest.
Facebook vs. Google: Visits
Facebook vs. Google vs. Yahoo: Visits
Closeup of Visits: Facebook Passes Google
Facebook vs. Google vs. Yahoo: Uniques