Using a snapshot of Web traffic from December, Compete's director of online media and search, Jessica Ong, found that 15 percent of traffic to major Web portals like Yahoo, MSN and AOL came from Facebook and MySpace. The lion's share of that traffic, 13 percent came from Facebook.

Google, which has profited handsomely from directing Web surfers to their destinations during the past decade, was third with 7 percent, just behind e-commerce site eBay, which had 7.61 percent. MySpace was fourth with just under 2 percent.

Surprise gain

The numbers proved eye-opening because Google used to dominate most Web-referral categories. "I was surprised to see Facebook has become No. 1," Ong said.

In other categories, Compete's data showed Mountain View's Google still on top, but Palo Alto's Facebook was not far behind. For example, Google accounted for 21.3 percent of referrals to sites catering to movie fans, but Facebook was second with 12.4 percent. And in a video category, Google - which owns YouTube - was first with 22.9 percent, but Facebook was next at 12.7 percent.

Facebook's meteoric growth as a Web destination was a factor. Facebook says it has 400 million active members, including about 225 million added in just the past 12 months. Its size now rivals that of major Web portals and its demographics mirror those of the Internet in general, Ong said.

"Putting all this information together, we can say that Facebook has become an integral part of the consumer Web experience, similar to how portals like Yahoo and MSN are part of most consumers' online sessions," Ong said. "So the message for the advertising industry is that more serious attention needs to be paid to social-networking sites like Facebook, and advertisers need to figure out how to leverage this traffic."

nothing really new here, but the pace in which FB is growing and shifting more and more traffic is impressive. okay, one little surprise: Even only at 2% I wouldn't have thought to find Myspace in this list at all.