Is that really different than the fellow who only watches FOX News or the fellow who only watches MSNBC?

I don't know if your question is genuine or whether you were more asking out of anger...it's different from the standpoint of editorial involvement. Fox News and MSNBC have editors who filter the majority of their content, while the users (readers) contribute, "feedback" in terms of what stories are, "hot/trending" as well as comments. In Twitter/Facebook these roles are reversed, with the users filtering the majority of the content, barring outright obscene content.

Fox News and MSNBC do have the option of pushing stories which change the conversation, thereby controlling how things are delivered on a story-by-story basis, like an art form whereas Facebook has dials and levers to tweak particular aspects of how sharing is done. I would say news content on Facebook is 90% sharing, 10% curation while news content on MSNBC and Fox are 80% curation and 20% sharing.

Or the deep social implications of creating self-reinforcing bias loops in the news those 1.5B read, fake or otherwise.

A Devil's advocate might say you need to balance the good of social networks with its evils

Or a product that has positively impacted the lives of most of those 1.5 billion people, as evidenced by the fact that they use it daily.

All the research on the matter I've seen showed that people who use Facebook weren't any more susceptible to fake news than people who didn't.

But if you get your news from traditional media and not social media, you're less likely to be exposed to blatantly fake news and therefore you're less likely to believe fake news.