I think the reason is (d), that most employees believe in the company's mission and do not think they are working for an unethical company, but instead that the company is being unfairly portrayed.
Most in Facebook or most in general in tech companies? If the latter, I'd bet my life that most people couldn't care less about visions or missions, which is more the realm of Founders, very young employees and marketing materials. I'm sorry to state it that harshly...
