Several people have asked me about this recently, thus I am putting up a small post about it.
Adademia.edu is designed for social networking with an academic/research focus. In some ways it is similar to facebook, in that you make connections with others on the site, can post things about yourself, and state your interests… but on this site you would post a paper you have written, or state your research interests rather than your romantic ones. People can follow you and “like” your most recent paper. You can also add teaching documents, and a CV.
You can find others with similar research interests. They have a fun research interest tree, for example, planetary science is located under Earth Science and Physics>>Astronomy and Astrophysics. You can also add subfields to these headings (e.g. planetary tectonics, mars).
My favorite feature is that they will notify you if someone searches the web and finds your academia.edu page. They tell you what the search terms were, what country the person is searching from, and what time they searched. I find it quite fascinating to see what (sometimes seemingly random) searches lead to me!
They have an example profile here: http://oxford.academia.edu/RichardPrice