It's perfectly fine to use the metric in question if its definition is clear. However, the definition was apparently not clear and Facebook therefore misled ad

It's perfectly fine to use the metric in question if its definition is clear. However, the definition was apparently not clear and Facebook therefore misled advertisers. Metrics are at the core of Facebook's business. If they don't get this right, that's fairly embarrassing, and raises a lot of questions about their other metrics. Perhaps this was not outright fraud but even if it was mere incompetence that's bad enough.