Google's decision to remove its invite-only registration process for Google+ turned out to be the right one.

Once Google+ opened its gates to the public last week, the social network exploded to 15 million visits from 1.1 million the week before-a 13-fold spike in traffic. The social site jumped 1269 percent in market share growth, and analyst Paul Allen believes Google+ may have already reached 50 million members, and is acquiring around 2 million new users each day.

Google+ may be hot again, but with Facebook ready to roll out massive changes of its own, how long can Google+ keep users interested?

Released back in July, Google+ initially burst out of the gates, attracting 20 million visitors in its first month. A social network for Google made sense, after all, since it could take advantage of Google's entire family of cloud products and Internet applications like Google Documents, Calendar, and Reader.

At its launch, Google+ had features that Facebook didn't have at the time. For instance, instead of one massive friends list, users could create different Circles to be able to exclusively follow or share information with a specific group of people. This was a sea change from Facebook, where any status update could be seen by any friend.  

Google+ also trumped Facebook in one other key area: Video chat. In Facebook, users could only chat with one user at a time; on Google+'s Hangouts, users can start a hangout, and invite several other friends to join. The video switches from person to person based on who's talking, but with the ability for users to mute and unmute one another, Hangouts was definitely one of the best features of Google+.

Among its other features, Google+ also came with a completely different photo album platform where users could easily see recent photos uploaded by friends, and Sparks, Google+'s recommendation engine for finding interesting articles, videos, and photos.

The goal for Sparks, Hangouts, Circles, and everything in Google+ was to instantly connect people everywhere to what's most important to them, according to Google's VP of social business Vic Gundotra.

After a few months, however, interest in Google+ began to wane. By the end of August, traffic had slipped to 1.1 million visits per week, and people were spending less time on the site, too.  

Once Google+ effectively ditched its invite process and added its slew of updates, including screen sharing collaboration for Hangouts, the floodgates to Google+ reopened. But now that the 90 day field trial is over and the real Google+ is here, can it possibly sustain its massive amount of traffic?

Originally, Google+ was going to be the Facebook killer, but based on the changes announced at Facebook's f8 event, it doesn't look like Facebook will die out anytime soon. Last week, Facebook unveiled its new Timeline profile pages, which essentially turn profiles into automated scrapbooks of one's life, and application support for dozens of new companies and services. Since Google+ launched, it has also cleaned up its News Feed, added a way to share updates with specific people (a la Circles), and has added video chat capabilities (though not group video chat).

Facebook is also expected to announce its long-awaited iPad application at Apple's media event on Oct. 4, where many believe the world will get its first glimpse at the iPhone 5.

Unfortunately for Google+, Facebook isn't going anywhere. If anything, these recent changes to Facebook's interface may even solidify its standing as the world's most dominant social network. Google+ will need to develop more integration with the other Google platforms in order to really be a powerhouse. Adding a search bar last week was a good start, but they'll need more to compare with the likes of Facebook.

Even with Google+'s traffic spike on Tuesday, Facebook still had 72 times more visits in the U.S. on the same day.