You're right that Tumblr's social aspects are more developed than Posterous's and that this has been a huge driver of their growth but to me that's not so much solving two different problems as Tumblr being more smartly designed -- realizing that blogging, even minimalist blogging, is inherently social.<p>As for Tumblr's top tier investors -- sure, that's true. I was trying to emphasize that Posterous, on paper, has everything it takes to win.