In 2010, Facebook accumulated some 8.93% of site visits online, while Google secured just 7.19%. However, YouTube took in 3.52%, and so if you combine Google properties they are in aggregate more popular than Facebook.
Facebook took third place in 2009, making its sharp rise to the top a surprise. Perhaps it was expected to edge up one spot this year, but to take first place is a move that few anticipated.
Here is the top visited sites according to Hitwise for both 2009 and 2010.