https://www.cnet.com/news/facebook-looks-to-snail-mail-to-confirm-ad-buyers-are-american/

Facebook hopes to stop foreigners from meddling in future US elections by verifying their location via postcard.

The postcards will contain a specific code required if a campaign mentions a specific candidate running for office. It won't be required for issues-based campaigns. Once the buyer receives the postcard, they'll be able to use the code to verify their ad campaign on Facebook.