According to a study released Friday by web-tracking firm comScore, the web search giant’s new social networking tool, Google Plus, has amassed 20 million users in just three weeks since its launch on June 28.

While that figure is still far from the 750 million registered users of Facebook, it is still enough to make Google Plus the fastest growing social network in history. The spectacular early success indicates that Google Plus may well challenge Facebook once it comes out of a trial mode in which Google has strictly restricted the number of users, Comscore said.

“It would be difficult to think of many sites that reached such a large number in such a short period of time,” said study author Andrew Lipsman.