FWIW I actually agree with you on this point. It's a bit of a prisoners dilema problem though. Basically everyone at this point has "defected" (reported signups). To be the only company to "cooperate (report active users) would be bad PR in a world in which most people don't pay attention to (or in many cases even understand) the difference.

It sucks a little, but I'm not sure what to do about it.

-harryh, engineering lead @ foursquare

All you want to do is showcase your company's growth in the best possible way without being misleading. They're not being misleading or shitty. If you're a free service that doesn't require a signup and makes money off advertising, then reporting registered users doesn't make sense. Advertisers just want to know eyeballs, so Google Analytics Visitor numbers will do. For Foursquare reporting registered users doesn't really make much sense either. They could have easily reported mobile installs too, but that would be similar. Internally, I'd be checking other growth metrics such as "How many people are using Explore", or "How many people are referring friends", or "How many businesses are getting new leads as a result of Foursquare". These are much more important questions, but alas, getting the media to care about that is hopeless.