One of the challenges of social media, one of the reasons clients often have difficulty in being able to pay for social media is the lack of standard metrics. Remember: it's not that organizations don't want to use social media. They often don't know how to justify allocating resources towards social media.
BtoBonline ran a sidebar, "Common metrics for social media" that provides a list of metrics to evaluate. Unlike newspaper circulation or broadcast viewership, there are still no standards of success.
And some of the metrics may not apply to every situation, client or campaign.
But it is at least useful to start.