From the analyst covering "They’ve warned about a decline in one of the basic fundamental financial drivers." which was "changes in the fourth quarter that cut the number of viral videos in people’s news feeds. Those tweaks, and others, have already reduced time spent on Facebook by roughly 50 million hours a day, he noted."
I am going to assume that FB is smart enough to have figured out how to trim the 50m hour engagement # from an ad cohort that is likely not that profitable or important.
Because guess what - the brand advertisers that elevate overall CPM already don't want to be in your shitty viral cat video, or your stupid publishers-gaming-FB with autogenerated content.
I would go a step further and predict that within this quarter we see an internal FB study for advertisers suggesting that their algorithm that trimmed that 50m in hours has actually IMPROVED the value of the advertising by a commiserate level.