Large number of users is not an excuse as several businesses use Messenger bots to handle common customer queries and handover to humans for escalation.
Heck, I myself have developed Messenger bots with over 100M conversations through them.
So actual `humans_needed_for_support` would be much lesser if Facebook used their own product.
I agree that there are many tools including bots that can cut that number down. Every time you're prompted with a bunch of articles before you submit a ticket to any help desk, it's part of that (given your bot reference I'm fairly sure you know this).
The numbers are a fantasy scenario anyway, even 10% of that (600 support staff) is not small, and it's not a cost Facebook wants to bear.
They should do support, but then, they should do a lot of things that they don't do.
Heck, I myself have developed Messenger bots with over 100M conversations through them.
So actual `humans_needed_for_support` would be much lesser if Facebook used their own product.