I guess the work contract also said that the Facebook employee has to show up for X hours per week at Facebook HQ to fulfill their duties. In other words: the permanent home office arrangement requires a new work contract anyway.
there is no "work contract" for employment in the US. It's at-will employment, the employer can change the terms at any time and if the employee doesn't like it, he can go elsewhere. Not saying this is a good thing (European countries do have work contracts for instance), or even expressing a preference (I personally think having a contract would be a good thing), just stating the state of affairs in US employment.
