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Facebook is now the top source of traffic for major news and entertainment portals such as Yahoo and MSN, according to traffic analysis firm Compete, and is “among the leaders” for other sites as well. Although far from conclusive, this is just another sign of how the “social web” is becoming an increasingly dominant force in terms of driving traffic flows on the Internet — and that in turn makes it a growing threat for major web players such as Google (s goog), MSN (s msft) and Yahoo (s yhoo). If your core business depends on controlling and/or getting a piece of the web’s traffic flow, as it does for all of those companies, the social web is something you ignore at your peril (which helps explain the launch of new services like Google Buzz).

Compete’s director of online media and search told the San Francisco Chronicle that a snapshot of web traffic from December showed 13 percent of the traffic to major web portals like Yahoo, MSN and AOL came from Facebook. Traffic from Google generated just 7 percent, which Compete said actually put it third in traffic sources behind eBay (s ebay), which accounted for 7.6 percent.

It’s important to note that Compete’s analysis is just another data point, and probably shouldn’t be taken as definitive. The jockeying for top spot as the web’s No. 1 traffic source has been going on for some time, and every measurement firm has its own numbers, which often conflict with each other because of differences in their methods. But there’s no question about the overall trend: Facebook has been growing strongly in terms of overall traffic to the site and the traffic that it drives to other sites.

Steve Rubel of Edelman Digital says that he sees Facebook becoming an increasingly powerful competitor to Google. “I see Facebook starting to look more like Google while Google tries and stumbles at becoming more social,” he writes. “Social networking is here to stay. It’s where attention spirals are flowing and no one looms larger than Facebook. And while Facebook has plenty of critics and they run into the occasional privacy concerns, I believe that they will dominate the landscape the next few years.”

Billionaire entrepreneur and sports team owner Mark Cuban noted a similar phenomenon in a blog post last year, saying the traffic coming to his blog from Facebook and Twitter was increasing while the traffic flow from Google was “declining significantly.” He called this phenomenon “huge, because of the behavior implications for users, and because of the business implications for Twitter, Facebook and Google.”

It’s worth pointing out that while Facebook may be driving more traffic to portal sites and to blogs — particularly those like Perez Hilton, which gets far more referrals from the social network than it does from Google, according to Hitwise — that doesn’t mean it’s going to replace Google any time soon. And Google, along with Microsoft and Yahoo, is doing its best to integrate social web content from Twitter and Facebook into search results, through indexing arrangements with those sites. But the balance of power is definitely shifting.

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