Now that almost everyone is on board with Social Media, it'll be treated with the same level of scrutiny as other channels for marketing. Television gets measured, even if imprecisely. Radio gets measure, despite the difficulty. Direct mail gets measured precisely because it is precise. Google search ads lend themselves to very targeted measurement.
You've got your work cut out for you if you don't know how to prove ROI. As a marketer, you no longer have to spend most of your time convincing everyone else in your organization that social media is a serious and permanent part of the landscape. You asked for it. Now you have to deliver.