The top two spots are still held by Google (970 million worldwide visitors) and Microsoft (869 million worldwide visitors), respectively, though Facebook is rapidly gaining on them. Microsoft has clearly been growing much more steadily in recent months; Google's traffic seems to be flattening out. Facebook is undoubtedly growing the fastest while Yahoo simply isn't growing at all. It looks like the top three spots are going to be changing in the next few months. How and when they change might end up shocking some people.
Three months ago, Facebook passed Yahoo to become the second largest video site. Two months ago, Yahoo CEO Carol Bartz admitted that Facebook could one day become Yahoo's number one competitor.