Ivan Booth has an excellent post about outcomes for Social Networks - it's organizing, not mobilizing.
What it all comes down to is that we're focusing on organizing people into a permanent anti-genocide movement (and much of that happens in a decentralized, self-organized sort of way) rather than simply mobilizing people for a particular event or campaign and then sending them home. If you just need bodies at a rally, names on a petition or donations in your coffers, mobilizing through traditional means will work great. But if you need an active, educated and effective movement, organizing through social webs has the potential to create much more lasting change.
Now couple what Ivan is saying with Wendy Harmon's advice on to begin to collect and reflect on your social media experiments and measure them little by little, adapt some of Charlene Li's case studies for big brands and spice it up with metrics from Dave McClure linked to strategy and calculate with Frogloop's spreadsheet. Bake for 60 minutes in a logic model.