Consider looking at the underlying goal of a company policy, not the particular technology it wants a policy to address. For example, if your policy said, 'Companies cannot use the phone/meet for lunch/sit next to each other on the subway to make disparaging comments about their employer'-- you'd be hard-pressed to defend that as a wise and sound policy. But the end result of a Facebook post is the same; only the technology is different. Sure, a post on a Facebook wall is more public than a Facebook message, but so is talking loudly on the subway so others can overhear. So long as the law is clear that the NLRB can protect employees' right to kvetch, I don't see that doing so on Facebook is substantively different than what's come before.