Last week, after a three-month "invite only" period, Google+ opened its doors to everyone, and new users flooded the fledgling social networking site. According to one analyst, Google+ has hit the 50-million-user mark in record time, further convincing those eager to be convinced that it will eventually challenge Facebook's seemingly unbeatable dominance. Here, a brief guide, by the numbers:

10 million
Number of users Google+ reportedly had in July

50 million
Number of users Google+ likely now has, according to according to Paul Allen, an unofficial statistician for Google+ and the founder of

Number of days it took Google+ to hit the 50-million-user mark. It's hard "to imagine a scenario where Google+ doesn't end up with hundreds of millions of users," says Allen. "It's just a matter of time."

Number of days it took Facebook to reach 50 million users

Number of days it took MySpace to reach the same milestone

2 million
Number of users per day that have been signing up for Google+, according to Allen. It "doesn't hurt Google+ any that Facebook can't seem to stop tinkering with its interface and has less and less respect for its users' privacy." says Steven J. Vaughan-Nichols at ZD Net. "If I were Mark Zuckerberg, I'd be worried. Very worried."

Percent growth of Google+ from the week ending September 17 to the week ending September 24. On September 20, the social network began allowing anyone to join, following a three-month period where membership was by invite only

15 million
Number of visits to Google+ last week, making it the eighth most popular social networking website, according to Experian Hitwise. "Even with this data, it's still too early to tell if Google+ is going to be a real player," says Trefis Team at Forbes.

Google+'s previous ranking among social networking sites, according to Experian Hitwise 

33 million
Number of visits to Twitter last week, making it third among social sites

530 million
Number of visits to YouTube last week, making it second among social sites

1.8 billion
Number of visits to top-dog Facebook last week

800 million
Number of users Facebook has. Even "if Google+ adds 1 million users a day for a year, which would be astronomical growth even for Google, they still wouldn't be halfway to Facebook's numbers — and Facebook isn't going to suddenly stop growing," says Forbes' Trefis Team. "I still don't think Google has come up with any massive innovation other than not being jerks about data ownership… and maybe that's enough."

Sources: Experian Hitwise, Forbes, Los Angeles Times, NBC Bay Area, PC Mag, ZD Net