Advertising on Facebook is a specific, paid mechanism for delivering messages to an identified target audience segment, that is completely distinct from their social content sharing features. Heck, you might not remember it, but there was a time when there was no advertising on Facebook at all.

Regulating that specific mechanism is well within the purview of the FEC, and can be done without interfering with the ability of Facebook users to share content with their own network of connections.

I suggest that it is very possible that Facebook, with its access to something like 1 billion MAUs, needs to be regulated, and regulated possibly differently than companies before it. The FEC has some rules for political ads with respect to the Internet, but I am in favor of revisiting those to see what rules need to be established for the greater good of the public

>There are no easy answers to "keeping propaganda off Facebook"

It appears that Russians bought $100,000 worth of ads, so that is some place to start looking that is wholly different than a discussion on politics of users.