How effective is ResearchGate as an academic selection and recruitment tool?
Some mathematicians I know (me included) appear in Research Gate. Many many others don't. As such, in the areas I'm related to, RG is not a significant source of information. It might be significant with respect to a single person who worked on her/his profile, but not representative of the market. When I click on "related researchers" on RG, the omissions in the list are way more significant than the inclusions. 

More specifically to your question, I didn't know that RG had any relation with recruitment. We wouldn't use it because it is not obvious that our target audience is well represented in it.