Well, if you have a Facebook account, you know how much time you spend on Facebook and you also know how much time others are spending, right? Now we have statistics that show that Facebook is the 2nd most popular site from a total number of visitors perspective.
Compete Inc, (a company which publishes research data on these kinds of interesting web & other stuff) published results that showed that Facebook's 134 million unique visitors in January bypassed Yahoo's 132 million unique but was just short of the 147 million unique visitors on Google.
Another statistic reported by Compete was that users spent 11.6% of their web time on Facebook which is more than double the time they spent individually on Yahoo or Google.
I think that Facebook's popularity is largely due to the connectivity it offers which no other site currently offers. Even Google Buzz just doesn't have the same "connectivity" and interactivity which I feel Facebook offers.
Will Facebook bypass Google in traffic sometime soon? Well, given the January 2010 numbers reported by Compete Inc (shown above), I think that that could happen sooner rather than later. It's not just the sheer volume of users interested in networking with family and friends that will make Facebook the number one site. I think it's also because Facebook offers personal and business interactions; what with the fan page and group participation opportunities that no other site offers. LinkedIn and Google's Buzz have a few features that may grow over time but I feel that they have a while to go before they catch up with Facebook.