I'm not too familiar with research gate, but for the rest:

Quora: By far the worst when it comes to objective facts. While there are gems here and there, most upvoted answers read like pep talks, and usually don't have much content beyond that of a decent pep talk. Go here if you want a pep talk or a popularization of some scientific concept.

Stackexchange: is where you want to be most of the time. The answers are good, and their accuracy is much better than anywhere else on the net. If you're asking about a topic anywhere close to mainstream in academia (or if you're interested in computers), you'll find an expert on that very topic.

Lesswrong: Honestly, lesswrong is too small. Unless you're going to ask about niche topics (EA, Singularity, Cryonics, and the like), there's a good chance nobody here is an expert on what you're interested in. If you are interested in the niche topics, you'll find people willing to give you detailed answers. But beware of only getting one side of the story. The answers you'll get don't necessarily reflect expert consensus, or even rationalist consensus (See yvain's opinion polls and how they match up with the comments you read. Also, check out all the other rationalist hubs. For instance, the tumblr rationalists have a markedly different atmosphere.).