Quis custodiet ipsos custodet? (Who watches the watchers?)
To have some kind of filtering or curation, you necessarily have to put some people into a position of power where they have the ability to censor this noise. On a forum where it's explicitly known that such "moderators" are present, and even better if their biases are known, this is perfectly fine. If I'm on a science-oriented discussion forum and it's advertised that the moderators of this forum will ban anyone who attempts to spread pseudoscience or anti-science religious junk, that'll help attract me because I don't want waste my time looking at that crap.
There's also forums where there's very little such curation, and instead community moderation is mostly relied on: users are given moderation points to boost or suppress other posts. That's fine too: you know going in what the deal is, so you just have to see if you mesh with the other members or not.
However, if a forum claims to be "open" and "inclusive" but then censors people, that's just hypocritical and wrong. If you're going to push a bias, then you have to at least inform users of this.
As it relates to Facebook, which is supposed to be for everyone and not just people of a certain political persuasion, they shouldn't be censoring anything unless they're going to make this known up-front (which they haven't). "Trending topics" should be anything that's highly popular. If some completely wrong and debunked BS is the most popular new story among your site's users, then you need to show that at the top, or else you're not serving your users. If you don't like that, then you either need to inform them of how you're going to censor and slant things, or you need to get rid of users you don't like. Or maybe just get rid of the "trending topics" section, and replace it with something more fine-tuned so that one group of users doesn't get annoyed at seeing junk they disagree with that another group of users really likes.
The simple fact is that you can't please everyone. If you try, you're going to fail. Sites like Reddit avoid this by having tons of different sub-forums, each with their own rules and moderators. Some far-right conservative wackos can co-exist with far-left liberals there because they're on separate subreddits.