https://phys.org/news/2018-05-facebook-election-related-issue-ads.html

Such issue ads played prominently in Russia's efforts to interfere in the 2016 U.S. elections. Twitter and Google also have been working to prevent elections interference, including by labeling political ads as such and requiring advertisers to disclose their funders.

Facebook says that when users click on the label on such an ad, they will be taken to a page with more information, including who paid for it and how many people saw it.