This is a naive view. "Mainstream media" isn't just cable news, they get are all on Twitter, Facebook, Reddit, etc. They often have a large presence on these platforms and the directly influence the conversation around stories.

Facebook and Reddit are not publishers of news so that comparison makes no sense.

That's a distinction without consequence. Your comment is about media companies "dictating" things; social media just trades the editorial desk for Facebook's opaque newsfeed algorithm, or the faceless Reddit hivemind, or Twitter's endless sea of bots. All that matters is reach, and social media has traditional media outnumbered and outgunned.