Tumblr—social media's next advertising platform [Upstart]

Tumblr is a shadow figure in the social-media world. Quantcast
calculates that Tumblr receives 44 million visitors each month, but
the three-year-old social blogging platform still ranks behind
Facebook (Nasdaq:FB), Twitter, and Pinterest in popularity with
users—and with advertisers.

http://upstart.bizjournals.com/companies/media/2012/10/08/tumblr-analytics-ma...