You don't need to cover billions. I can't remember the exact figure, but I saw a study once that said the vast majority of content on social media is created by a surprisingly small number of accounts.

A number of other accounts just retweet or share that original content, and most people don't really contribute at all, they're just there to read.

Even if you focused your attention exclusively on accounts with more than a million users, say, you'd catch nearly all of the worst stuff and stop it from spreading too widely.