Readers per URL among blogging services: is it important?

Microsoft Live Spaces has something like 75 million spaces, and about 125 million unique visitors a month (at least those were the last numbers I saw shared).

WordPress.com, according to Automattic’s CEO (that’s the company that makes WordPress), Toni Schneider, in an interview I had with him yesterday, has 400,000 WordPress.com blogs, but has 25 million unique visitors per month.

Which service is going to be more attractive to advertisers? Which one builds a better business?

I wonder if anyone is doing a comparison of readers per URL among all the blogging services (and MySpace and FaceBook too)? Anyone doing research on these kinds of metrics?