Great article from Liz Gannes on AllThingsD in January 2012:

The site really took off was when the curators — people who primarily respond to other Tumblr users’ content by “reblogging” it on their own pages — came on board.

Today, creators are probably 10 percent of Tumblr, and curators are 90 percent, Karp estimated.

And further, the average post on Tumblr is reblogged nine times, he said.

That can mean a single post gets the amplification of nine separate blogs, nine RSS feeds, nine blogs’ page views and followers, and nine bloggers’ syndication on Facebook and Twitter. “So your content has a huge footprint,” Karp said.

In January 2012, Tumblr was doing 15 billion pageviews a month across 41 blogs.

Says About Tumblr, the average Tumblr user creates 14 original posts each month, and reblogs 3. Half of those posts are photos. The rest are split between text, links, quotes, music, and video.

Liz Gannes reports that Tumblr is still much bigger than Pinterest: http://allthingsd.com/20120629/whats-next-for-pinterest/

"ComScore replied that Tumblr still has the edge, though it’s close in the U.S. As of May, Pinterest had 20.3 million U.S. monthly unique visitors, while Tumblr had 25.3 million."

"Worldwide, Pinterest had 31.9 million uniques in May, while Tumblr had 69.7 million."