You sue the person who made the post.
What, are you gonna ask the authorities to extradite him into the US to show up in his appropriate court date?
Everyone knows that you pay offshore troll farms for your negative reviews / disinformation these days. EDIT: Heck, there's a huge market for fake offshore accounts. You can buy fake offshore accounts and do the negative trolling yourself these days. (3rd world citizen "farmed" thousands of accounts on Facebook, to bypass the moderators. 3rd world person sells those accounts to someone nasty, and then the nasty person uses the account to troll / harass / misinform)
Its a well known fact that moderators mostly look at new accounts. 3rd world citizen actively using an account and look vaguely "humanish" for 30 days is enough to bypass the bulk of Facebook moderation.
You ain't winning this battle. Not with the current set of laws, not with the current set of techniques being used. You have a better chance of suing Fox News / CNN. Seriously.
Basically allow suing a class as a whole, where if the class loses each member (named in the suit up front) gets a lien?
Or perhaps if you can identify the tree of dissemination, you can roll up all the harm from each individual actor and apply it against a higher level instigator to get the entire pyramid (up to that instigator’s level) worth of damages out of one actor.
Swatting and Doxing are two kinds of content that must be moderated, and I believe we can morally say that these two behaviors must be stopped. Revenge porn maybe as well, and some other rules of the internet.
I don't know where to go for "fake reviews" or how to police that behavior. I simply am not imaginative enough to come up with a law or rule that's useful. Its clearly damaging behavior. More of a thing on Amazon than Facebook really, but it still happens on social media (Facebook reviews, Yelp).