An employer cannot inhibit the formation or function of a union but are under no obligation to provide the facilities or platform to form one. What FB have done is akin to not allowing an organizer to use the company photocopier to print union leaflets
But they can’t target union talk specifically:
Companies may prohibit workers from talking about nonwork issues in work areas during work hours. ... In other words, a company can prohibit extraneous conversation, but may not prohibit only union discussions while allowing employees to talk about anything else they wish.
Source: https://www.nolo.com/legal-encyclopedia/shop-talk-rules-unio...