Seems like they’d have a vested interest to claim anything as daily users. Bots, trolls, whatever.
Privately held company numbers on the other hand are generally much more suspect (Uber before IPO, Snap before IPO, We work, Theranos...)
But there are plenty of proxy measures that show similar growth. For example the AppAnnie "active user" measure actually measures on device usage, and shows that 4 out of the top 5 most used apps world wide (or maybe ex-China? Unclear) are Facebook-owned (WhatsApp, Facebook, FB Messenger and Instagram. WeChat is number 4).
https://www.appannie.com/en/insights/market-data/state-of-mo...
Even if Facebook is shrinking, I don't think WhatsApp and Instagram are, and who knows what other companies Facebook may be able to buy up rather than be overtaken by.
Their report indicates they just passed the 2.5 billion user mark.