WebProNews

Comscore Media Metrix just released audience numbers for some of the largest social networking sites on the Web.

Here's a list of some of these sites. All figures are unique visitors for May in millions. The numbers are quite sizable. Moreover, comScore's CEO sees no slowdown. "This is a phenomenon we're seeing not only in the U.S., but also around the world," he said. "The challenge for social networking sites will now be monetization and how advertisers will respond to the global marketing potential of these sites."

Amen.

MySpace (51M)

Facebook (14M)

YouTube (12.6M)

MSN Spaces (9.5M)

Flickr (5.1M)

Yahoo 360 (4.9M)

LiveJournal (3.0M)

(MySpace and Microsoft are Edelman clients)

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Steve Rubel is a PR strategist with nearly 16 years of public relations, marketing, journalism and communications experience. He currently serves as a Senior Vice President with Edelman, the largest independent global PR firm.He authors the Micro Persuasion weblog, which tracks how blogs and participatory journalism are changing the public relations practice.