I will be on a panel at tomorrow's Innovation Summit 2010 discussing just this.... Measurement. The ultimate goal for any marketing campaign is turning relationships (via social media or otherwise) into sales. If this doesn't happen then most likely the head honchos at your company will pull the plug and say that marketing doesn't work. (Trust me, I have been there...)
So in order to prove that what you are doing online is working, you HAVE to show it....
From a 2006 DoubleClick report: Survey respondents said they were twice as likely to notice a web ad , not click on it, but visit the advertised site later (61 percent), as they were to click on an banner ad to reach a site (30 percent).