By definition, don't all contractors have to generally not work from an office? Otherwise it's very easy to deem them full-time employees and they are entitled to benefits. Not sure of Facebook's specific situation or what roles they are contracting, but I'd imagine most of them aren't at HQ.
They're just permatempts. Every company has them these days.
The content moderators specifically work out of an Accenture office and aren't really contracted to facebook. They are Accenture's w-2 employees.
These contractors at tech companies typically are full-time employees, they're just full time employees at other companies. The setup is that Facebook, or Google, has a contract with company like Accenture, which in turns provides its own full-time employees to do some work on FB/Google projects.
