Yahoo knows, and it might be bigger than what you think. That’s according to a study that Yahoo commissioned and released at Web 2.0. Here are some numbers: Just 12 percent of the online population is aware of RSS, and just 4 percent says they use RSS. So far, those numbers track what other studies have found.

But then Yahoo dug deeper and asked people who used “personalized start” pages such as MyYahoo, MyMSN, Google’s personalized home page or Bloglines to describe what they saw on their pages. From that, the group Yahoo hired, Ipsos Insight, concluded that 27 percent of online users use RSS and don’t know it. Which means, according to Yahoo’s study, 31 percent of online users consume some form of RSS. “The majority don’t know they’re using it,” said Scott Gatz, Yahoo’s senior director, personalization products.

Other interesting findings:

– The “aware” RSS users are, not surprisingly, mostly males. The “unaware” RSS users, however, skew toward females, 55 percent to 45 percent. They are also more evenly distributed across the age range, whereas 50 percent of “aware” users are clumped in the 18-34 age range.

– 28 percent of online users are aware of podcasting, though just 2 percent listen to them.

There’s more. We’ll try to get a digital version that we can post for folks who like this kind of stuff. Download the pdf of the report here.