Truth is, social media has direct and indirect effects that are impossible to measure perfectly. But so are many PR and marketing efforts! While you can't measure all SMM effects, you should still measure the ones you can.
setting your conversion goals.
Track conversions.
Assigning monetary value to each conversion by estimating a Lifetime Value of a Customer.
Collect incoming traffic and conversion numbers using Google Analytics. Compare with the monetary value of those conversions.
Determine the cost per channel by adding labour and other costs.