At first, Facebook planned to rely purely on its algorithms to populate the section, but it faced criticism for failing to sufficiently focus on big stories like the protests in Ferguson, Missouri (unlike on Twitter), while highlighting Ice Bucket Challenge videos. So, it roped in human editors to curate the section.
The company faced a larger firestorm in 2016 over accusations that editors were suppressing conservative-leaning news stories, which Facebook denied. At the time, CEO Mark Zuckerberg said the company was committed to "building a platform for all ideas. Trending Topics is designed to surface the most newsworthy and popular conversations on Facebook."
Just a few months before the 2016 presidential election, Facebook switched from humans to algorithms for the descriptions for each topic, though editors continued to pick which items were featured on the list. Phony stories still made it to the section, though, such as one about broadcaster Megyn Kelly supporting Hillary Clinton in the election.
Despite more recent efforts to make the feature relevant, Facebook finally killed it this past June, as the company focuses on other news features like a breaking news label on links, a local news tab, and news videos in the Watch section. By the time Facebook wound it down, Trending Topics was still only available in five countries and drove very little traffic to publishers. So not only did the feature cause extensive controversy, but barely anyone was using it anyway. It's hard to imagine anyone will truly miss Trending Topics, as it certainly caused more trouble than it was worth.