Here is one from originally 2016 https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3033144

Apparently - I did a quick search - conversion rate at facebook is said to be on average up to 9% (in marketing materials) or around 4-5%.

In this study, the authors show conversion rate consistently sub 1%, which is however not the point of the article. Rather, they also show that the lift in conversion (so the actual impact of the ad) as calculated by observational measures usually over-estimates the actual lift (sometimes significantly). Observational measures were those used by advertisers on facebook - maybe even now?

The more snarky comments from the presentation are missing from the draft, of course.