In Western society maybe but in emerging markets Facebook is still huge. And for many em small businesses it is their only online presence.
Americans and Canadians earn Facebook over 4x per user what Europeans do and over 10x what Asia Pacific users do [1]. Facebook's user base shifting overseas implies a massive revenue drop (and corresponding adjustment to value).
[1] https://www.statista.com/statistics/251328/facebooks-average...
That doesn't sound unrealistic.
Assuming the tenth Asian user (a) is as lucrative as the first and (b) costs 1/10th as much to provide services to as an American user, sure. I'm doubtful about those assumptions. And this is all just for treading water.
In India, for instance, Facebook is especially popular among old people like my parents, or poorer people. The former won't spent much money online because habit. The latter don't have much money to spend.