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Google+ continues to set records as the fastest-growing social network in history, but Google's social juggernaut is beginning to show signs that it's losing steam.

Ancestry.com co-founder Paul Allen (not to be confused with the Microsoft co-founder of the same name) posted his most recent analysis of Google+'s growth on his Google+ account Tuesday. According to his analysis, the search giant's Facebook competitor will likely reach 18 million users by the end of Tuesday, but its growth rate has dropped by 50% from its peak.

"Last week we saw two days where more than 2 million signed up in a single day," Allen said in his post. "If that rate had continued, Google+ would have reached 20 million users by last Sunday night. But the last four days have averaged only 948,000 new users, and yesterday the site added only 763,000. Yesterday's growth of 4.47% was the slowest viral growth since Google opened up invites back on July 6."

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Why is Google+'s growth slowing down? Google Trends indicates that the buzz around Google+ has died down some, which is natural for any major news item. Allen makes the important point that Google+ hasn't been promoted by any of its other properties and that the social network is still invite-only. Once Google+ is promoted on YouTube or on Google.com, its growth may simply skyrocket.

Allen estimated that Google+ hit the 10 million user mark sometime on July 12 or 13. Google CEO Larry Page confirmed that Google+ had more than 10 million users during an investor earnings call on July 14. Its most followed user, Mark Zuckerberg, now has more than 250,000 followers, despite not posting a single public item on his Google+ account.







Graph via Paul Allen's Google+ page