According to Hubspot, marketers should be measuring social media marketing campaigns to determine what is making money and what isn’t. In order to generate a positive ROI with social media, Hubspot has created and refined a metric they call Value of a Like (VOAL). Here's the formula:
L (Total Likes)
Likes on a Facebook page or number of Twitter followers
Unlikes and unfollows.
Average number of times you’re posting links.
C (Average Clicks)
Average number of clicks on the links to your site you’re posting.
CR (Conversion Rate)
Average conversion rate of your website, from visit to sale or visit to lead.
ACV (Average Conversion Value)
Average sale price or average lead value.
Hubspot has also built an easy to use calculator at ValueOfALike.com. Answer six questions and see what each Like or follower is worth to your company.