Ok, maybe the first question is what is Tumblr? Tumblr is a microblogging platform and social networking website (now owned by Yahoo!). The platform allows users to post multimedia and other content to a short-form blog. The system has built in methods to share with other users – driving a ton of engagement and visibility for folks who utilize it. They also have a really nice mobile app.
While I don't frequent Tumblr, I do share our content on my Tumblr blog through the Jetpack integration that WordPress added. I also follow a few Tumblr's and get a really nice update email from the system with a breakdown of what's shared by whom. As a marketer, you may not realize how big this network is, though. The numbers are pretty impressive!
Website Host Reviews has shared this infographic all about Tumblr: