I don't disagree, but I don't think Facebook is responsible for 'fake news' or even 'hate news'. As with any community there are actors within it which self select to group together and communicate. Now what it does do is allow what might be a dispersed group to become a 'local community' but Facebook's role in that is providing the access, not in influencing what those people do or say.

One need only step between rabidly conservative and rabidly liberal enclaves on Facebook to see different groups surrounding themselves with their own messaging.

To date I don't see Facebook exerting an editorial influence that is swaying meaningful sized groups. Sure there are groups that use Facebook to do that, but it isn't something you can buy or arrange for from the company.

See the comments below regarding Facebook's algorithms about what gets presented in people's feeds. Facebook is absolutely a principle player in what people see, by virtue that their algorithms can be gamed in particular ways, and their promotion of news and products can be bought. To think that Facebook isn't selling outsized influence over people's feeds for obscene amounts of money would be naive.

EDIT: I believe this is also the secret to Reddit's success...

I see it differently. I see people gaming Facebook's algorithms just like people have worked to game search engine algorithms for years, in order to push their message in front of people who would be receptive to it. I don't see Facebook as the agent of that gaming which is perhaps an insignificant point as far as some in the discussion would consider but for me it is the essential point.

I see Reddit, Facebook, Topix, Google+, HN, all providing "watering holes" or a place for a community to form, and these days there is always an algorithm connected to a mechanism for the community to use to signal its approval and disapproval. In "broad" community sites like Facebook and Reddit there is an explicit 'hands off' policy on how those communities operate (until there isn't). That supports a wide number of communities and optimizes for high user counts. In "narrow" community sites like HN and various forum sites, there is an explicit 'hands on' policy which drives the community to a particular place. That optimizes for content/discussion that is highly aligned with the owner's editorial goals even at the cost of ejecting users who don't align with those goals.

The way I think about it, for a multi-community site to actively "influence" its users, the site operator would have to apply some mechanism for shaming or promoting specific communities within it. Further it would associate users with that level of compliance which would signal to them if they were 'betting better' or 'being worse'. The signalling is needed to help the users understand which way they are "supposed" to go, without that signalling it is much harder.

So multi-community sites don't (and won't) do that because it results in users leaving and taking their eyeballs with them. And ultimately the point of such sites is to collect as many people there as possible with as wide a demographic as possible, so that people can advertise goods and services to them.