Here you will find stories from contingent workers at Facebook. None of them are employed by Facebook on paper; they are employed by contracting companies, staffing agencies, vendor companies, and such loopholes. However, they work for Facebook and at Facebook sites and offices.
Unless stated, their roles are not temporary; even though they may have signed a fixed-term contract, it keeps getting extended. Sometimes, they will have to "take a break" for a few months before coming back to work at Facebook, so they can continue to appear temporary.
Unless stated, the names of the contingent workers and any details that may identify them have been changed or obscured for their privacy and protection.