Is Academia.edu useful?
I distrust it for an entirely different reason. I once wanted to download a paper, and could only do it if I signed up. I signed up by logging on with my facebook account... Well, academia.edu took my profile information, and, without me knowing it, created an academia profile. With my picture, publications it could find via search engines, and a list of interests that were half right and half ridiculous. I'm a neuroscientist; it listed me as being interested in marketing, among other things. I only discovered this profile about a month later. 

Apart from it being entirely unprofessional, I simply do not trust information that is on there, as I have first-hand experience that information on me was wrong.