Guns give people a sense of power they wouldn't otherwise have. That is something that makes guns so appealing. People loved guns and purchased several of them before facebook and they will purchase their guns after Facebook. Facebook is just one of millions of sites on the Internet, I suspect the change won't even be noticeable when Facebook eliminates gun advertisements.
Eliminating gun advertisements on Facebook and Instagram will no prevent users from purchasing guns. Our culture has way too much exposure to guns in television and movies for it take any dent in the purchasing of guns. Gun owners have a gun for a false sense of security not because they see an ad on Facebook.
Nothing will get rid of guns in today's societies. Although guns are portrayed in social media in certain advertisements, the removal of them won't get rid of guns. In fact, if guns were outlawed completely, more people would feel compelled to have them and powerful for being in possession of them. The gun debate is a difficult one, but outlawing advertisements and actual guns will not solve the problem.
No, eliminating gun advertisements on Facebook and Instagram would not prevent users from purchasing guns, because someone who wants to buy a gun isn't going to be swayed either way by something they see on facebook. For something like gun purchases, the demand is inelastic. People want them, facebook or not.