To reduce costs, difficulties with licenses and to make the scientific discussion possible for everybody, scientists and institutions are moving to the fast developing Open Access sector. Scientists are often valued by the amount and quality (where it´s published and the amount of citations) of their articles.
While the amount journals that provide an open access option have increased in recent times, it is substantially more expensive to publish an open access paper.
To include this concept platforms like researchgate and scienceopen are using structures from social networks.
I cannot speak about the platform scienceopen, but researchgate requires an affiliation to an institution (past affiliations might work if they can be verified, although I am not sure on this), and a lot of the time the institution will have journal subscriptions negating the need to send article requests on researchgate.
In addition to the various open access platforms you listed (thanks for that!), there are numerous members here who have an affiliation to an institution, and while I cannot speak for the others, if a member is looking for a paper they cannot access they are more than welcome to send me a PM and I will try retrieve it.