Answer: by creating an archive for all advertisements on its platform.
Facebook will create an archive that contains all ads put out by a certain page, so that users can see all the content that an advertiser may be running. Additionally, Facebook will inform users if they are among an ad’s targeted audience. These rules will apply to all ads, not just political ones. There will be additional rules for federal election ads, and will include information on the amount spent on the ad, the number of views it has received and demographics of the viewing audience.
“Transparency helps everyone, especially political watchdog groups and reporters, keep advertisers accountable for who they say they are and what they say to different groups,” said Rob Goldman, Facebook's vice president for ads, in a blog post on the announcement.
The move comes just days before Facebook, along with Twitter and Google, is scheduled to deliver testimony on foreign interference in elections before a Senate intelligence committee.