According to a recent Nature survey, ResearchGate wins in quantity and Twitter stands out in terms of communication quality.
I have maintained my profile and uploaded papers to Academic.edu, which is not as popular as ResearchGate, but it is a lot less irritating. I too find those automated emails that profess to come from colleagues active on the site annoying. Academic.edu is much less invasive by comparison, and I don't mind getting an email from them saying "This number of people searched for you just now."
I am sticking to Academic.edu for now, because for social scientists, usage of the two sites was more closely matched, according to the survey. Also it has much higher web traffic than ResearchGate overall and is open to anyone to join, not just scientists.
In a long run though, I should start up a Twitter account at some stage...
Update: Some believe though science communication and research productivity are incompatible, as reported in this Science piece on "Who are the science starts of Twitter?"