Web giant's social-networking experiment doubles the number of members in the past week, according to ComScore data.
Google's latest social-networking foray has reportedly hit the 20 million member mark in just three weeks.
Google+ has added 20 million unique visitors since its release late last month, including 5 million from the U.S., according to data released today by research firm ComScore. Google apparently reached that milestone just a week after Google CEO Larry Page reported that the social network had reached 10 million visitors and had received a "ton of activity" with more than 1 billion items shared and received each day.
"It is definitely the fastest ascent to 20 million visitors that I can think of," Andrew Lipsman, ComScore's vice president of industry analysis, told the Los Angeles Times.
A Google representative declined to address the report, saying the company does not comment on third-party metrics.
Google launched Google+ late last month not even in beta, letting in only a handful of users, who could later invite their friends and family. Page noted at the time that the service remains in a "field trial" mode, and with "a lot of barriers" to use it right now, but that the company is continuing to expand how many have access to it.
Despite its meteoric growth in its first three weeks, a population of 20 million pales in comparison with Facebook's 750 million, half of whom log in daily.
Google's first major foray into the social networking world came by way of Orkut, a service that has seen some success outside the States but has largely been ignored by U.S. users. Google tried its luck again with the launch of its Buzz social network, but the service came under fire from users who criticized the service for violating their privacy by automatically making some of their contacts public.