Trending Topicsgate was a tempest that occurred in May 2016, when a content moderator who worked for Facebook’s Trending Topics feature styled himself a whistle-blower, opened up to Gizmodo, and said that Facebook employees were suppressing conservative news.
To elaborate on my first annotation to this: I think what happened with "Trending Topicsgate" in May 2016 was not good for a few reasons. First, it made people worry about how much power social media like Facebook has. Some said Facebook workers were stopping conservative news from being seen, and that's not good because it might change what people think and believe. Second, it made people think about fairness. If Facebook was really stopping some news and not others, that's not fair. It means they might be picking sides, and that's not what people expect from a big platform like Facebook. And lastly, it made people fight more about politics. When something like this happens, it can make people who like different things not trust each other, and that makes it hard for us to talk nicely to each other. So, "Trending Topicsgate" was not good because it made people worry about how fair and honest social media is, and it made political arguments even worse.