Google+ now has more than 62 million registered users and is adding more at the rate of 625,000 a month.
According to Paul Allen, who's been tracking the site's success, membership is likely to hit 400 million by the end of next year.
"It may be the holidays, the TV commercials, the Android 4 signups, celebrity and brand appeal, or positive word of mouth, or a combination of all these factors, but there is no question that the number of new users signing up for Google+ each day has accelerated markedly in the past several weeks," he says.
Indeed, says Allen, nearly a quarter of the site's users have signed up in ther last month. He says that if the present rate of signups continues, numbers will reach 100 million by the end of February and 200 million by August.
"I expect the growth to continue to accelerate however. Google can continue to integrate Google+ into its other products and word of mouth will continue to build," he says.
"Most importantly, 700,000 Android devices are activated daily and this will become a very significant source of new users for Google+. That number will also grow next year."
These numbers are good - but not as good as Facebook's 800 million users. It's also worth noting that the Google+ figure is for total users, rather than active users. Recent figures from data analytics firm Chitika indicate that many of those who signed up for the site in the early days are now not bothering to use it.
But Paul Allen reckons that the site is ready to take off, hitting a critical mass that could give it the mainstream acceptance of Facebook.
"It won't be long before new users start encountering family and friends as well as the thriving tech and media sharing community that embraced Google+ early on," he says.
"I predict that 2012 is going to be a breakout year for Google+ and that it will end next year with more than 400 million users."