I didn't actually go to the Stewart-Colbert rally, but I have been fascinated by the way that our generation has been particularly drawn to comedic news commentary shows as legitimate sources of news media. I think that the turnout at the rally is a prime example of this. This article on CNN sort of questions whether or not the rally was an effort to boost voter turn out. I do not think that was the purpose of the rally, in spite of Glenn Beck's nervous comments.
In terms of the Stewart-Colbert rally, social media played a huge role in getting the word out. Their facebook groups for the events were the primary event invitation for the gathering. Each page boasted tens of thousands of RSVPs for the event. While this form of social media was effective in getting the word out for people to go to the rally it may not be effective in getting people to go to the polls. Stewart and Colbert's fans do tend to be of the demographic that support Obama, but I think the gathering was less about using social media to promote a political agenda than it was to provide a response to the Glenn Beck rally. In this sense, social media was used to mobilize a demographic block different from those who turned out for the Glenn Beck rally. Facebook probably would not have been an effective means for Glenn Beck to get people to show up to his rally.