I respectfully and slightly disagree with Prosperina. The ruling from the NLRB Administrative Law Judge in the Hispanic United case -- the decision described in the article she referenced in her thread post -- did conclude that employees' Facebook postings concerning workplace conditions constituted protected concerted activity under Section 7 of the National Labor Relations Act.
However, nothing in the OP suggests that the original poster had a "conversation" of any stripe regarding work issues. Unless the original poster had such a conversation, neither the NLRB's recent Facebook cases nor Section 7 would appear applicable to his situation.
All workers, irrespective of whether they are unionized or not, have the right under federal law to discuss work conditions amongst themselves. If two or more employees engage in such discussion via Facebook or other electronic means, said communication is likely protected under Section 7 of the NLRA.
In this instance, though, it is not at all clear the original poster had a conversation with anyone concerning the site supervisor at the prior post. Consequently, (absent the existence of a union contract or some other contractual protection), his employer may be able to discipline or terminate him for allegedly "insulting comments" posted on the Internet to no one in particular. The key here may turn on whether he made said comments in the context of an electronic conversation with another employee.
One final point: This Section 7 protection covers employees nationwide, not just in the Empire State.