Facebook reported fake numbers to advertisers

The issue here is the “potential reach” calculation when setting up an audience/targeting for a campaign. It has nothing to do with the billing for actual impressions. If you say you want to target “English speakers in the US” the potential reach may say 200M. However your ad spend limits and frequency and bid options and ad creative ultimately determine who sees the ad at what price, and generally prevent you from ever hitting that max reach anyways. Apparently that potential reach calculation was known to be off by some percentage (maybe overestimating potential reach). It has no bearing on what you get charged as an advertiser and I doubt this disrupted any campaigns.

The issue here is actually that the "potential reach" was inflated by a large number of accounts which Facebook knew were fake, but didn't care to remove. Those accounts were not distributed evenly across all targeting parameters, which means advertisers were setting up their audiences differently than they may have otherwise.