Would it really damage advertising revenues that much? If the fear is that Google would be hit by large lawsuits from some twelve year old posting a music video on YouTube, or that Facebook would be liable for a link to it, I expect that media conglomerates would be happy to give de facto immunity, so long as Google or whoever is willing to put Viacom's commissars in charge of which content is allowed and which isn't.
Obviously it's good that Google is on the side of angels on this, but if it's actually in their best interest is a relevant angle for analysis. It also would tell us how hard they're actually willing to fight to block SOPA.