Users on the website have been calling for a 'dislike' option on posts - the closest update so far was adding to the 'like' reaction with a number of other emojis - but it appears this latest development will work slightly differently.
While some people have speculated it could be used in a similar way to Reddit's ranking system - which allows users to react positively or negatively to each post - The Verge reports that it will likely work as a moderation tool.
It has been suggested that if the 'downvote' button is launched, it would initially be restricted to public posts to let users flag up "offensive", "misleading" or "off topic" comments.
The downvote view count would not be visible to other users, and a spokesperson for Facebook has confirmed that the testing is being run on a small number of users.
A statement read: "We are not testing a dislike button. We are exploring a feature for people to give us feedback about comments on public page posts.
"This is running for a small set of people in the US only."
It comes after Facebook CEO Mark Zuckerberg admitted he had made lots of mistakes over the last 14 years since launching the social network.
He said: "Over the years I've made almost every mistake you can imagine. I've made dozens of technical errors and bad deals.
"I've trusted the wrong people and I've put talented people in the wrong roles ... I've missed important trends and I've been slow to others. I've launched product after product that failed."