The LinkedIn is more generic for industry and academia as a social media and emphasizes your potential for employment. ResearchGate is a little less generic for industry and allows members to post their research papers. This in turn helps these members show their potential for possible employment.

ResearchGate may not yet be the equivalent of LinkedIn but it is getting there. ResearchGate is better than Academia.edu in that you can collaborate and use more features for free. I really like the report from ResearchGate about who has been reading your articles and where in the world they are located. You can create projects to group your articles on a topic and invite others into that project to collaborate. I like the Q&A feature of ResearchGate that lets people share expertise and resources.