There is also selection bias in whom Facebook will show the ads to due to Facebook's CTR models, other bidders' bids, and who is browsing Facebook more to see m

There is also selection bias in whom Facebook will show the ads to due to Facebook's CTR models, other bidders' bids, and who is browsing Facebook more to see more impressions in general. They should have worked with an ad platform to run the ad as a PSA so they could use the ad platform's own randomization instead of relying on having the auctions they win result in anywhere close to a usable sample.