Engagement numbers are averaged across Q4 2017, I assume. Zuck announced the news feed algorithm changes in mid-January 2018. Now, they were likely A/B testing it for part of the past quarter, but it seems quite likely that these changes were not 100% live for all of North America for the entirety of the past quarter.

Therefore, speculatively, engagement in Q4 was likely flat or declining across NA already. What is more uncertain is the order of operations here. Did they publicly announce these changes in advance of the earnings report to get ahead of bad numbers?