Academic networking can be a way to get access to material that you otherwise might not be able to get to. People will share publications on various sites, or – if you have connected with them – you can approach them to send you versions.
ASNs tend not to be as strict as going via institutional repositories (IRS) – like our own WRRO. That is good for access but often dubious for legality. Be aware that publishers are beginning to pursue sites for copyright breaches and the Academia.edu & ResearchGate have changed their terms and conditions so that this risk falls on the user. This is all slightly conflicted as Elsevier has issued takedown notices to ResearchGate but arguably indulges in the same behaviour on its own platform Mendeley.
If you want to be neat and tidy and ensure that everything is legal and REFable you should engage with your IR, or a disciplinary equivalent.
There is an argument for changing the environment through which research is shared and diminishing the monopolistic control of publishers. Open Access is attempting this at official level. Low level file sharing probably happens all the time, but a site like SciHub has hugely enabled this. Its status in the UK is considered highly dubious but it is effective in file sharing (albeit illegally). Its founder Alexandra Elbakyan is unrepentant and her views are at least worth considering:
“[Elbakyan says] ‘As a devout pirate, I think that copyright should be abolished’. [She] points to Article 27 of the United Nations’ Universal Declaration of Human Rights, which states ‘Everyone has the right to freely participate in the cultural life of the community, to enjoy the arts and to share in the scientific advancement and its benefits’. By rooting illegal pirating in a social justice narrative, it is easy to see why Elbakyan might be referred to as the ‘Robin Hood of Science’.” (Morrison et al 2017)