You realize the extent to which that chart is useless, right? My Posterous blog is at post.danieljackoway.com. Compete's not going count that towards Posterous's numbers.

I might be wrong, but it seems like Posterous users are more likely to use custom domains--Tumblr is more of a social network, so it makes more sense to be on a subdomain of tumblr.com (just as your facebook and twitter pages are on facebook.com and twitter.com). Posterous is more for stand-alone blogging, so having it be on your own domain makes sense.