When the history books are written on how the media shaped the 2008 presidential race, three big networks that reach tens of millions of Americans will have a central place. As recently as the 2004 election, though, two of them were virtually unknown and one hadn’t even been thought of: YouTube, MySpace and Facebook.

The online networks have turned into important mass-market distribution systems for political messages. But while the Obama and McCain campaigns have each in their way learnt to use the new “social media” for their own purposes, the ability of voters to interact with the political messages has had important results that have not always been obvious, according to media pundits.