"Please accept our apologies for the spam we caused this afternoon," Google head of social, Vic Gundotra, wrote on his G+ wall on Saturday evening.
"For about 80 minutes we ran out of disk space on the service that keeps track of notifications. Hence our system continued to try sending notifications. Over, and over again. Yikes."
"We didn't expect to hit these high thresholds so quickly, but we should have. Thank you for helping us during this field trial, and once again, we are very sorry for the spam," he wrote.
Since launching in a limited field test two weeks ago, Google+ has captured "millions" of users, Google chairman Eric Schmidt revealed on Thursday, according to TechCrunch. The social party began with mostly tech journalists and tech insiders, before Google had to shut down invites. Google briefly re-opened the social network to the public last Thursday.
Google says it has to throttle invites as it continues to fix bugs and ensure that its infrastructure is reliable and fast enough to handle scale. The gradual opening is a stark contrast to the "tens of millions" who joined Google Buzz within two days.
For more, see PCMag's full hands-on with Google+, the slideshow below, as well as 6 Things Google+ Can Do That Facebook Can't and Social Networking Showdown: 8 Facebook Features Google+ Doesn't Have (Yet).
On privacy, see Google+ Privacy: Has Google Learned Its Lesson?