Do they function as networks?

Networking has always been a fundamental part of academic life.  These sites work in so much as they facilitate that.  However, often they are replicating networks that exist elsewhere.  The network is the people not the platform via which they communicate.  If you are interested in connecting beyond the Academy then these sites may not do if for you.

There tend to be a lot more profiles, than active profiles so check the activity levels of the people you are interested in before moving.

The important thing is to be where your peers are.  Very broadly ResearchGate has a higher proportion of Scientists and has more Social Scientists and Humanities Scholars.  Obviously, Humanities Commons focuses on the latter.

See the graphs in this article for more detail

Bardacki & Arslan (2017) “The majority of the participants (68, 85.0%) had never been invited through an ASN to collaborate.”

Thelwall & Kousha (2014) “ reflects scholarly norms, rather than being a typical general social network site dominated by younger members, because faculty (who are presumably typically older than students) get more profile views than do students”