I have accounts of both LinkedIn and Academia.edu but have rarely made use of them. LinkedIn from my perspective is a useful platform to present a professional profile as it is widely used by both researchers in academia and practitioners in the industry. However, as an early-stage PhD student went straight from my undergraduate and Master to a PhD programme, I have, on one hand, very little work experiences but a few short-term internships, and on the other hand, no acknowledged research work. My LinkedIn profile is not at all likely to capture eyes of potential employers. Moreover, because I had little work experiences, I almost have no connections on LinkedIn, which even worsens the situation. Therefore, I still need to contrive to build up my competitiveness and network.
Academia.edu is specialised for academics. Since I intend to pursue a career as a researcher, Academia.edu is probably a networking tool I should pay extra attention to. Albeit I still have no published paper that I could upload, I found it very convenient and helpful to follow some renowned scholars who are expertise in my research topic. Following their works will no doubt allow me to keep up with the most up-to-date research that is associated with my own research interests. Nonetheless, I do find that it have been a practical problem for to spare enough time to have a look at the papers uploaded by scholars that I follow.
I agree that LinkedIn and Academia.edu are better online platforms for professional activities than Facebook. I personally consider Facebook as an everyday social network as their functions are designed for this purpose while Facebook is largely used as an everyday social networking tool.