Gundotra posted the news on his Google+ profile today, adding that Google+ also has 100 million monthly active users -- if you count in the mobile app.
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"This week we also hit an important milestone--over 400,000,000 people have upgraded to Google+. It was only a year ago that we opened public sign-up, and we couldn’t have imagined that so many people would join in just 12 months," wrote Gundotra.
For comparison, Google+'s biggest competitor, Facebook, had 955 million active users in July.
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Of course, the methodology of counting active users (as opposed to the total number of users who've signed up for the service) between the two companies may differ, but judging by these numbers, Facebook is currently approximately 10 times bigger than Google+.
That may sound like a big advantage for Facebook, but one also has to take into account that it took Facebook several years to reach 100 million active users, while Google+ managed to do that within 12 months.
This story originally published on Mashable here.