I agree with your take on mindset. But eyeballs isn't what makes the company successful in the end, the revenue does. And who has/will have more revenue is an open question for now. Personally I'd put on Posterous.
You're right -- obviously this isn't a post mortem or anything. The trends could reverse, and Posterous could become wildly profitable with less users while Tumblr has plenty of traffic but no money.
Although it seems pretty obvious that both are headed for a combination of advertising/freemium model, and the revenue that comes from that seems pretty correlated to userbase size.
I'd argue that Tumblr, by focusing on the social, has opened itself to a wider variety of possible revenue sources. They could succeed at social advertising where Facebook has failed. (Not that Facebook doesn't make money, just that they do it while looking like idiots to users who see mismatched contextual ads.)
