Not exactly apples to oranges, because that doesn't make the community argument invalid. It's the fun of exploring content on Tumblr that made it possible to grow a network of erotic photoblogs (and not only erotic, check all the 'f*yeahsomething' categories, all kind of lols and such). I didn't bother to check if Posterous' TOS somehow block such content, but still if it was there, currently it's not so easy to find and follow as on Tumblr.