Earlier quoted context omitted.
HN has addressed this problem by not granting new users flagging privileges - you need a certain karma threshold to flag an article or comment. And flagging/vouching privileges can be removed for HN users who abuse them.
Yeah but you can just buy warmed-up verified accounts on SEO marketplaces, often several years old and patiently curated to be blandly inoffensive and ready to be turned into whatever you need.
Another potential mitigation might be to put a limit on the number of posts that a single user can flag in a day. At some point, the cost of large-scale content manipulation could be made to exceed the expected gains.
It may even be profitable for Facebook to crack down on this. Every celebrity post that gets illegitimately taken down has potential for showing ads to millions of people.