"A {LIKE} is a form of social currency". What does this mean?

What is currency used for? ofcourse to trade, yes?

Similarly, in a social sphere, getting a like is like getting a dollar(i suppose) and each like makes you socially richer. 

Ideally you would see posts with high likes on news feeds and this increases the outreach of the post. just how we could do more ads with money, we get more reach with likes...