This is a dated sentiment. According to this Forbes article Tumblr is on track to make $100 million in revenue this year.
http://www.forbes.com/sites/jeffbercovici/2013/01/02/tumblr-...
Tumblr has actually managed to pull off ads very, very tastefully. With brand and product recognition and well thought out ad-campaigns that feel like a part of Tumblr rather than banner ads.