The IAB have tapped up some of the great and the good of the social media industry to look carefully at social media measurement, you can find the site here: IAB social site. As attendees of Will McInnes‘ MeasurementCamp would expect there is no silver bullet, but then there never has been with a lot of marketing disciplines anyway.
Instead they have provided a collection of smart questions to shape the thinking right at the beginning of a campaign so that measurement can be baked in.