The 6 corporations "owning the media" concept came out just shortly before, and during, the rise of social media (e.g. I recall it circulating as far back as 2010-'12). In fact, reddit and social media are how a large number of people became aware of it.

Without picking a side or making an argument, it's basically a repeatable factoid that gets people wondering how much collaboration, and potential illicit collusion, is really being applied to the stream of messages they are exposed to on a daily basis.