"How to Calculate the Value of a Like"

In this recent blog post on the Harvard Business Review, Dan Zarella (who works for HubSpot) seeks to quantify the value of a Facebook "like" with the following formula:
L/UpM * (LpD x 30) * (C/L) * CR * ACV = Value of a Like
Where...
L (Total Likes): The total number of audience members connected to your social media account. On Facebook, these are Likes of your page, and on Twitter, these are followers.
UpM (Unlikes-per-Month): The average number of fans who "unlike" your social network account each month. On Facebook, this is an "unlike," and on Twitter, this is an "unfollow."
LpD (Links-per-Day): The average number of times you're posting links, and potentially converting links driven from your social media account. On Facebook, this is the number of posts you're making, per day, that lead to a page on your website. On Twitter, this is the number of times, per day, you're Tweeting these kinds of links.
C (Average Clicks): The average number of clicks on the links to your site you're posting on your social media accounts.
CR (Conversion Rate): The average conversion rate of your website, from visit to sale or visit to lead. This can be an overall average, but for increased accuracy, use the conversion rate measured from traffic coming from the social network you're calculating.
ACV (Average Conversion Value): The average value of each "conversion." In this context, a "conversion" is the action you've used to measure CR for. It could be average sale price or average lead value. For increased accuracy, use the average conversion value of traffic coming from the specific social network.

While measuring the ROI of Social Media is still in its infancy, I thought this article was interesting and further information can be found at ValueOfALike.com.