Of course it is Facebook (hmmm..since they trademarked "face", can they stop everyone from using facebook in forums, on sites and blogs?
Last time I checked, the traffic going to facebook was more than the traffic going to google.
Facebook all the way !!
well Facebook and it will take GOOGLE+ atleast 5 years to get to them i mean if the can provide so unique features. other don't have any chance not even twitter. It's just my opinion
Stumbleupon is one of the big players as well (obv nothing close to FB's traffic).