Facebook doesn’t prohibit firearms posts, only sales. Hate speech, threats, and violence are also naturally prohibited.

Lots of folks post quite regularly regarding firearms. I try to limit my own exposure, as I’ve been a part of professional interview processes where potential employee social media is snooped, and have been on hiring teams to see firsthand that a great deal of emphasis can be placed on any particular political leaning, but I do know many, many of us post firearm photos, participated in shooting sports groups, follow firearms businesses, etc. Just can’t sell firearms through Facebook.