http://www.quantcast.com/tumblr.com

http://www.readwriteweb.com/archives/tumblr_leaves_posterous_in_the_dust.php

From the RWW article: "Fred Wilson, whose VC firm is an investor in Tumblr, claimed in April that "Tumblr is more of a social network than Posterous." While Posterous would debate that - it too enables users to follow people, comment and re-blog - the mere fact that Tumblr has many more users than Posterous makes it a more powerful social network."

Any ideas on the fundamental reasons Tumblr is crushing it and Posterous isn't?