There is no way Facebook could employee enough people to check all the posts.
Facebook alone had over 2.3 billion users, in the 4th quarter of 2018.
They had over 1 billion users who had posted within 30 days.
If those people only posted once per day ... that would still require checking over 33 million posts/day.
If Facebook were to hire people to check 1 post/minute ... based on an 8 hour/day ... they would have to employ over 68,000 people just to check posts.
THAT is why they need to rely on AI.
And that's just one social media site.