Tumblr already has the attention of almost everyone under the age of 18, so imagine if Tumblr started to implement some more social features into the site. If Tumblr could play up being the 'hip new thing' to Facebook's 'old and professional'... They might have something.
Is Tumblr the internet's sleeping giant?
UPDATE: Heres some food for thought; I run a relatively "cool" blog, and my follower counts are as follow: Twitter - 1950, Facebook 'Fans' - 3,850, RSS - 4,500 ... Tumblr - 7,500 (it's up 1,000 since last week). That also translates to Tumblr being my third largest referrer after Stumble Upon and Twitter. Facebook is hard to quantify however.