By Dan Mitchell, contributor

FORTUNE — Whenever the subject of traffic or membership on social-media sites comes up, a caveat should be offered: it’s really difficult to tell which numbers to believe, if any. Facebook claims 800 million “active users,” but I know lots of people — myself included — who have more than one account. So, active accounts, maybe. But the number of users is not likely to really be approaching 1 billion of Earth’s 6.8 billion inhabitants.

The numbers are even murkier when it comes to Google Plus. Since Google GOOG and AOL (AOL) combined. No wonder Google is putting so much money and effort into social media. Too bad it happened so late.

Still, Google is not easily dismissed. Too often, Google Plus vs. Facebook is presented as a zero-sum game, as if the success of one must result in the downfall of the other. But there’s no reason that they can’t both thrive, competing with each other (and with Twitter, et al) for users and ad dollars. They share many of the same users now, and that will likely continue to be the case. It’s easy to imagine them each claiming different demographics to appeal to different advertisers. Perhaps Google Plus will end up being the network of professionals, techies and creative types, while Facebook continues on as the network of the mainstream. Maybe Google Plus will end up with fewer members and less traffic, but higher CPMs.

Anyway, that’s one scenario. One big risk for Facebook in what is fast becoming a real competition is that it will lose more members the more often it creates obnoxious features like the Ticker, carelessly invades users’ privacy, or radically revamps the site, as it has done several times recently, culminating in the new Timeline feature. The more friends someone has on Google Plus, and the more features that site adds, the lower the cost to users of switching over from Facebook, or at least of adding Google Plus and going to Facebook less often.

Unless it is careful, Facebook will, if anything, be more prone to muck about with the site once it has billions of dollars of the investing public’s money at its disposal after its expected IPO this year. Better Facebook should use the cash to improve service and make deals with more media and gaming companies to add to the site’s features. That is, to work on making it more attractive to users rather than less so.