How many people are in your Circles (assuming you are in Google+ already)? There should be plenty: Paul Allen (not the Paul Allen of Microsoft fame, but the founder of Ancestry.com) believes there are 4.7 million users in Google+
(as of Saturday night, around 11 p.m., when he posted his estimate).
Allen describes his methodology as "based on US Census Bureau data about how many people there are in the U.S. with each surname." Basically, he compares the numbers users with particular surnames on Google+ with U.S. census data and then comes up with estimates of how many people there are on Google+.
It makes sense, and it isn't hard to get your head around. On the other hand, as he readily admits, it doesn't use global surname data, so its quite possible it doesn't accurately express numbers globally.
Allen plans to update his numbers on Monday, July 11, when he has completed all of his "surname counts."
Google is rumored to be readying a public rollout of Google+ by July 31. That's "by," not necessarily "on," so it could be sooner. For now, despite the estimate of millions of users, Google+ is still in limited field tests.
The uptake and excitement around Google+ means that it seems like Google has gotten a social networking service right (after failures like "Wave" and "Buzz.").