ResearchGate is exclusively designed for research community.  Its first and foremost objective of RG is to promote quality research. The options available in RG is to help the researchers to add and popularise their research findings. It is very much closely associated to other related research databases. Fruitful peer discussion on the recent research advancements are possible in RG.  Provision for reviewing and rating the inputs is the highlight of RG.

Other social media including Linked In are not like that. Any person can join them and and there is no rigorous monitoring in these social media. The fundamental objective of those is not the promotion of research, even though the details like achievements, publications etc.  can be included in the personal profile of an individual. 

Research Gate is for researcher community, while Linked in is for practitioners-professionals, each with different purpose, so it is difficult to compare.

This is a good questiion.

RG serves the needs of the research community worldwide.   By contrast, Linked in primarily serves the needs of the business community and job-seekers.

To agree with and expand (a tiny pit) on Pahlaj Moolio's reply, Facebook is vastly superior to both RG and LinkedIn in terms of being the "most useful, informative site" for communication with the whole world. Why? Because it has the most members of any social media site, it is the easiest to use to share the most common information (pictures, links, personal updates, etc.) and to the most people, it is free (whereas to use many of the most useful features of LinkedIn one must pay), etc.

So why do I check RG almost every day but rarely touch Facebook? Well, actually because of boredom or because I'm avoiding work I should be doing (or I can't sleep), but pretending that these weren't factors the answer would be because

1) I can't ask or answer questions of a technical nature on Facebook and expect an answer (although I don't really ask questions, but many people do and I do answer questions or try to).

2) I probably still belong to certain "groups" on LinkedIn (I don't know as I don't use my account), but I found that the more general nature of LinkedIn meant that it was harder to find interesting topics and even harder to find interesting topics that had interesting contributions. This is not so of RG.

3) Both Facebook and LinkedIn are social media sites that are bigger and more widely used than RG because RG is (as Pahlaj Moolio said) designed explicitly for a particular subgroup of professionals: researchers. Being a member of that subgroup, this is the better site.

4) RG is free. LinkedIn is fee only for a minimal account type and tries to rope you into paying for a various paid memberships.

5) The ability to share information through questions, answers, your personal page, and private conversations in RG is superior to LinkedIn, mainly because it serves a smaller, more close-knit community. There's often no point to "publishing" your research paper on LinkedIn rather than just listing it as part of your CV/Resume. The vast majority of members on LinkedIn are not going to care or understand most of the various papers accessible via RG.

I could go on, but the point is that "which is the best" depends on what you want out of the options. In this case, being a researcher, I'd rather belong to a social media site for researchers.

ResearchGate is exclusively designed for research community.  Its first and foremost objective of RG is to promote quality research. The options available in RG is to help the researchers to add and popularise their research findings. It is very much closely associated to other related research databases. Fruitful peer discussion on the recent research advancements are possible in RG.  Provision for reviewing and rating the inputs is the highlight of RG.

Other social media including Linked In are not like that. Any person can join them and and there is no rigorous monitoring in these social media. The fundamental objective of those is not the promotion of research, even though the details like achievements, publications etc.  can be included in the personal profile of an individual. 

I think that ResearchGate is only marginally a social media platform.  To function more truly as social media, it would require a timeline function that allows users to generate content and the ability to share posts and papers.  However, these things might reduce the emphasis on serious research.

It is interesting that academia.edu does have the ability for users to post, but it does not seem to be used much.  I think that this is because people do NOT perceive it as a social platform.

For scientific exchange of ideas RG is unparallel