Measuring Social Networking Sites

A colleague raised an interesting point in a meeting yesterday.  How would you assess the value of a client having a presence on social networking sites like Second Life or Facebook?   I must admit that, while I am a Facebook user, despite having poked around Second Life I am a neophyte on SL in particular.  Beyond looking at using Social Networking Theory, which could be a bit like smashing a fly with a hammer in this case, how would one undertake a sort of cost-benefit analysis (either qualitative or quantitative) to either 1) propose that a client jump in and create a presence or 2) evaluate the merit of having done so after some time has passed?   

UPDATE:  not 2 minutes after having posted this, a colleague came across this fascinating post outlining a Social Media Index.