:::::::This is just my simple minded opinion but I would postulate that Arbcom has no authority outside En-wiki other than to refer the issue to whatever Arbom would apply to that Wiki (like Commons). It could be argued that if that Wiki didn't have an Arbcom that their authority is inherited but I would argue that since the Arbcom members are voted in by the "community" the community of the applicable Wiki would have to at least vote to agree that an Arbcom decision would have cross jusrisdictional authority. With that said the Arbcom has made some pretty broad and vague decisions of late so they might well think that their decisions apply to Facebook or twitter feeds.