In the virtual world, just as in the physical world, different communities attract different types of people. These differences matter a lot to advertisers as they work to rationalize their social media spending. And they’ll be crucial to sites that hope to follow LinkedIn and Facebook with IPOs of their own.

Here is a snapshot of one month’s activity by U.S. users in several virtual gathering places, ranging from well-established players to relative newcomers. Key demographic information is shown below.

Sources: Google DoubleClick Ad Planner, Plus Demographics, Quantcast