I'm interested in repositories like zenodo, figshare, etc., where open-access papers (previously published) can be uploaded in order to improve the visibility of papers. Thanks a lot for your suggestions!
SSRN (https://www.ssrn.com/index.cfm/en/) can be an option.
If you want something similar to ResearchGate, you could consider Academia.edu. I personally don't like that site anymore though. They have become too clunky and push members a lot to get paid subscriptions.
Agreed with Dr. Wolfgang R. Dick Precisely, ResearchGate and Academia.edu are commercial sites, whereas most open access repositories are non-profits. These academic social networking sites have each raised large amounts of initial funding: $17.8 million for Academia.edu, and $35 million for ResearchGate.
Nevertheless, there are two main routes to open access are: Self-archiving / 'green' open access – the author, or a representative, archives (deposits) the published article or the final peer-reviewed manuscript in an online repository before, at the same time as, or after publication.
Andrey Davydenko, depends on what features you are keen on. I have used it for self-archiving some of my published work and it seem to have improved the visibility of those publications.
Farhan Azim : Many thanks for explaining this option to me, at first I had thought they only accept pre-prints. Indeed, SSRN accepts previously published papers according to what they say on their website. I uploaded some previously published papers to SSRN, let's see what happens.
Institutional repository could actually be used and this could be achieved via any library in such institution. This is because originally, libraries especially academic libraries are repository of information and knowledge that accrues faculty members of it's institution.
Repositories are several, remember that thanks to open science now the repositories have the purpose of disseminating everything that is in Open Access, depending on your specialty you could see that almost all universities already have theirs. In Mexico you can check it at: https://www.repositorionacionalcti.mx/. There are repositories by specialty such as NASA's for Astronomy, the National Seismological Service of Mexico for seismic issues, etc. But if your idea is to disseminate your work in an institutional repository, it would be wise to use those of your university.
University under which you study should be having a repository. Most of them have it already. But they do include mostly unpublished thesis. Check yourself before adding.