Last week, Google began allowing users to join Google+ without an invite. And according to the Hitwise data, this led to a massive influx of users all wanting in to the new service. From the week ending September 17th to the week of September 24th, traffic to Google+ grew 1269%. It received around 15 million total visits in the U.S. last week.
They also note that this data does not even reflect mobile traffic or traffic from the Google Notification Bar (Google's little black personalized bar).
This massive growth catapulted Google+ into the top ten of Experian's "social networking and forums" category. Before opening up to the public, Google+ was the 54th most visited site in that category. Now they are the 8th most visited site, just behind MySpace. Facebook, YouTube and Twitter are the top three in that category.
Google+ still seems to be dominated by what they describe as "influencers" and "early adopters."
Adding to the good news, "unofficial Google+ statistician" Paul Allen released his estimates regarding total users. He projected that Google+ has surpassed the 50 million user mark and that they are adding 2 million new members every day since allowing everyone to join.