To measure the ROI that you invested in social media, you can consider using tools like the Conversation Measurement Tool on Facebook and optimized CPM. You can also use Google Analytics or LinkedIn, which have their own analytics. The criteria remain same to track the traffic like clicks, shares, purchases, number of likes, etc. ROI metric may also include

☛ e-mail subscription

☛ Website blog traffic

☛ Webinar registration

☛ Registration for content downloads,etc.