Ancestry.com founder Paul Allen has been tracking the growth of Google+ for a while now and his estimates for the site's growth have been pretty spot-on from the beginning. According to Allen's latest calculations, Google+ now has more than 100 million users – up from the 90 million Google CEO Larry Page himself announced just 10 days ago. Allen estimates that Google+ is adding new users at a rate of about 750,000 per day right now. If this trend continues – and there is no reason to assume otherwise – Google+ could have 400 million users by the end of the year. That's about 50% of Facebook's current user base.

It is, of course, hard to tell how many of these users are active Google+ users. Chances are, only a fraction of those who signed up are actually regularly using the service, but Google has never released those numbers and isn't likely to do so anytime soon.

Thanks to Search Plus Your World and the fact that Google is integrating Google+ in virtually all of its services, chances are the service will continue to grow its user numbers, but as comedian Jon Stewart noted on the Daily Show yesterday, for quite a few Google users, Google+ is "as all Gmail users know it, what the f*ck is that thing up there. How can I turn it off?"