In fact, Karp revealed, if an original piece of content is published to Tumblr, that content is on average reblogged nine times, appears in four Facebook feeds, and five Twitter feeds. So among the 80 million blogs on Tumblr, only between 10 and 15 percent of the content is deemed “original.” The remainder is reblogged — something you can either interpret as the inspiring power of the Tumblr network or the slow death of original Web content.
This is a good piece, and it explains why Tumblr has yet to go down the road of monetization for individual users. Key line: "Because YouTube is sending a check to its users at the end of every month, YouTube’s monetization structure “shapes the behavior of the community” and normalizes content creation. It motivates content creators to churn out work for the sake of generating revenue on YouTube.