I am a big fan of social networks. First of all, when I try a social network, I look for the people that I am the most interested in following, ie top guns in the emarketing industry. I also pay attention to the curation potential of the site, ie the ease to find the information that matters for me. Finally, I look at the accessibility of the publishing tool, and how much I think I will be able to share with it.
Tumblr is an excellent website that gathers an enormous community, a great search engine, and a very smooth publishing tool. I've been playing around with it a bit lately, and found myself very satisfied by the user experience. If you'd like to connect with me on Tumblr, follow the link: Didier Gaultier.