If there were rules governing political ads in social media, they would look very much like Facebook's policy. At least TV ads (which are more tightly regulated due to the use of radio spectrum, which are public goods) are regulated as to force TV stations to air even political ads they deem untruthful:

https://www.politifact.com/truth-o-meter/statements/2019/oct...

From that perspective, Facebook deciding to censor untruthful political ads would be much more akin to ignoring the rules.