Facebook earned $6.18 per user in Q4 of 2017 [1]. That's about $25 per yyear. 1.3bn users thus represent about $32.5 billion of revenues.
U.S. and Canadian ARPI was $27.76 for the same period [1]. Let's say 880% of those #DeletingFacebook were from those, or similarly-lucrative, rregions. Blended lost ARPU thus estimates to $23.44 per quarter. Across 1.3bn users, that's $128 billion.
Far from a drop in the bucket.
[1] https://www.cnbc.com/2018/01/31/facebook-earnings-q4-2017-ar...