, I'm on LinkedIn, Academia.edu, and ResearchGate. For me, working in industry, LinkedIn is probably the best networking site, but I've found that ResearchGate is an excellent place to get answers to questions that arise in the course of my work. I"ve also been able to contribute answers to questions that others have, and I can get copies of papers that I need/want to read. I haven't found much use for Academia.edu. It is, as +Bryan Bonvallet
said, LI for academics, and isn't a great fit for me. Academia.edu offers access to papers, like ResearchGate, but I've found only a very few papers that I am looking for that are on Academia.edu exclusively.