There's literally a team at Facebook (Marketing Science), part of who's job is to tell advertisers what actually works and often, this can lead to them telling advertisers to spend less.

And this is an incredibly smart business strategy in that FB know (from experiments et al) that their platform works, and if they can show incrementality, then advertisers will invest more in the platform.

In game theory terms, it's a good strategy for a repeated game, which advertising definitely is.

Facebook does like money. Do you think lying to the biggest advertisers in the world is the best long-term strategy, or do you think instead it might be better to report out the most accurate results possible?