Value for the user. One Facebook "like" has various estimations of a price and some of them were quite hight (even going to thousands of dollars). A quickly found link from 2013 resumes some estimates: http://www.businessinsider.com/what-is-a-facebook-like-actua...
Have you looked at a stock chart of Facebook recently? All time high basically and worth $220b.
Or did you mean the value the company brings to the user?