I wish there was a rule that every chart used as any kind of argument must have both its axes described. If not, it's worthless. This one is. (Not to mention that you should also mention the source.)
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.
Your right on the data accuracy.
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