Most of the big social media platforms prohibit communicating about buying or selling firearms.
As part of our work at GunTab we monitor the firearm policies of major players on the internet. For example, we maintain a list of online payment platforms that prohibit firearm transactions. Spoiler alert, the summary is "all major payment platforms prohibit firearm transactions." While anti-gun policies aren't as universal in social media, they're not far behind.
We just finished a survey of the firearm policies of the largest social media platforms. They almost all forbid what we call "commercial firearm posts" - anything intended to facilitate buying or selling a firearm. Instagram, Pinterest, Reddit, YouTube all prohibit commercial firearm content entirely. Facebook theoretically allows commercial firearm content from federally-licensed firearm dealers (FFLs), but prohibits it between private individuals. While that's a more reasonable approach, it still prohibits commercial firearm posts that could be perfectly legal even if they resulted in a real transaction. Twitter is even more relaxed, allowing tweeting to buy and sell firearms, but prohibiting paid advertisements about firearms. So while social media is not as restrictive of firearms as payment platforms, it still bans the majority of commercial firearm content.
One platform that has taken a strong position of tolerance is the microblogging platform Tumblr. On its homepage it says "Put anything here you want". Technically it does prohibit content that involves trying to hurt people (things like "hate speech", "self-harm" and "terrorism"). But it does not seem to prohibit firearms. Nor does Flickr or Snapchat. Hopefully these remaining platforms continue supporting freedom of commerce and thought.
Keep an eye on the new list of social media platform firearm policies that we're talking about. We will continue to maintain it.