This probably falls under "nominative fair use", where use of a trademark to simply refer to a product is allowed. Given that the icons are clearly intended to

This probably falls under "nominative fair use", where use of a trademark to simply refer to a product is allowed. Given that the icons are clearly intended to be used as social media buttons, the de facto manner of linking to a social media site, I don't see selling them being a problem. From a copyright perspective, one might also argue that their conversion into a font is "transformative", but that's a much more subjective issue.