i thought the same thing at first, having just read the title, but it seems it's really not about the "social network" part at all. this is more like arXiv.org in that it's a platform for academics to publish their own research papers.
They have some interesting features (such as feeds of recent articles organized by journal you can sign up for), but by far the most important feature of academia.edu is growing academia.edu.
Having said that, I haven't found Academia.edu useful. I like Google Scholar better, and that has plenty of problems.