This is a cop-out for actually having to verify/review political ads.
Zuckerburg is really really good at the art of seeming like he is progressive but is empirically horrible for society.
That's ridiculous, it's not as though Facebook ads are the only source of political information in America. Even on facebook specifically that's not true; everybody's Facebook wall will still be packed to the brim with their friends, family and neighbors still talking about politics. We're talking about politics right now, despite there not being any paid promotional material present in this thread.
Advertisers simply do not fill the vital social role you seem to attribute to them.
Is he good at the art of seeming progressive?
He's hosted fundraisers for Chris Christie, his lobbying group FWD.us had created a GOP-led spin off called "Americans for a Conservative Direction" that supported the keystone XL pipeline, and the decidedly non-progressive politicians Lindsey Graham and Marco Rubio.
So when I hear people say stuff like "Facebook has a liberal bias" or that Zuckerberg himself is progressive, I'm genuinely puzzled.
What we perceive as fact is actually something derived from a lot of open discourse based on a lot of evidence. Removing open discourse based on whether the discourse is fact based is circular reasoning and a chicken & egg problem. It's not fact check, it just becomes outright censorship.