Final project: librarians on Tumblr

Technology: Tumblr
Context: As a tool for sharing library resources between librarians
Audience: Librarians and LIS students on Tumblr, "tumblarians"

I'm interested in researching how librarians are using Tumblr's social media space to share library ideas and resources with other professionals. Some blogs are official library blogs, while others are more personal. I want to see which blogs are more "successful." What kind of niche do public libraries with Tumblr accounts think they're filling, and which niches are they actually filling? I'm specifically planning on looking at which tags librarians are using to promote their blog posts.