https://techcrunch.com/2018/07/26/facebook-finally-hands-over-leave-campaign-brexit-ads/

The UK parliament has provided another telling glimpse behind the curtain of Facebook’s unregulated ad platform by publishing data on scores of pro-Brexit adverts which it distributed to UK voters during the 2016 referendum on European Union membership. The ads were run on behalf of several vote leave campaigns who paid a third company to use Facebook’s ad targeting tools.

The ads were run prior to Facebook having any disclosure rules for political ads. So there was no way for anyone other than each target recipient to know a particular ad existed or who it was being targeted at.