Google launched Google+ three weeks ago and now the social network has recorded 20 million users and marketing has not even started yet. On Wednesday, Web-traffic watcher comScore Inc. estimated Google+ has had 20 million unique visitors since its launch, including five million visitors from the U.S. A Google spokeswoman declined comment.

ComScore, whose estimates are based on a “global measurement panel” of two million Internet users, similar to the approach Nielsen uses to measure television ratings,doesn’t have data on the number of minutes people spent on Google+.

Still, the growth of Google+ has impressed observers because access to it is by invitation only, meaning people can join only if a current member invites them.

“I’ve never seen anything grow this quickly,” said Andrew Lipsman, vice president of industry analysis at comScore. The only other site that has accumulated as many new visitors in a short period of time is Twitter in 2009, he said, “but that happened over several months.” via Wall Street Journal

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