Facebook is not telling them they have to come into the office. Rather, it’s the contracting companies that these people actually work for. Quite a difference, which is clarified in the first paragraph.
Though that's one of the benefits of outsourcing: you get all the benefits of squeezing your workers to the maximum (which helps the bottom line), while still having deniability for PR purposes.
Facebook is responsible unless they loudly and publicly order their vendors to treat their employees better.
Yes, they are.
> A Facebook spokesperson told The Intercept that “for both our full-time employees and contingent workforce there is some work that cannot be done from home…for content reviewers, some of this work must be done from the office for safety, privacy and legal reasons,”