https://arstechnica.com/information-technology/2018/07/facebooks-downvote-system-begins-rolling-out-wider-in-us-heres-how-it-works/

Facebook's latest dalliance with a "downvote" button launched this week as a limited test in the United States. The feature began appearing on the service's mobile app without a formal company announcement—and we only found out about it by browsing on our phones.

The feature appears to currently be limited to "public" posts. Should your account be flagged for this week's test, every comment in a thread will include a numeric value and small up- and down-arrows connected to that number. Upon the first display of this Reddit-like change, the Facebook app will offer guidance: "Support comments that are thoughtful, and demote ones that are uncivil or irrelevant."