Tumblr.com points at the use of Quotes as an indentifier of "Popular" rank.
Tumblr.com has categories, one of them is Quotes, this to me is a sign that quotes have a higher than normal valuation with search engines.
Tumblr Defined: Tumblr is a microblogging platform that allows users to post text, images, videos, links, quotes and audio to their tumblelog, a short-form blog. Users can follow other users, or choose to make their tumblelog private. The service emphasizes ease of use.