A referendum is defined as a general vote by the electorate on a single political question that has been referred to them for a direct decision. An initiative is a petition that starts a referendum. Nothing like this exists on a federal level and only in a handful of states are citizens granted the basic right to actually effect the workings of their political system directly.The infrastructure and the technology exist that it is only by egregious oversight that it hasn't happened already.
People have lost and continue to lose interest in the political workings of this country because they feel impotent and disenfranchised. The very popularity of this site (a non - binding initiative site, if you will) is a testament to how a national referendum could re-engage them.