Those 800m users combined have more than 2 millions years "per day". They spend way more time brushing their teeths.
For just a second spent, "liking" something on Facebook seems a pretty efficient use of time.
Clicking "Like" has an unbalanced return on my time investment, short as it is. I get virtually nothing in return, but marketers and advertisers have another data point that they can use. My friends couldn't give a crap what I click Like on.
Well, in case of my friends Liking things, I do. It's a good recomendation engine. Especially, if your friends are not from the same industry as you, looking at what they Like is a way to avoid filter bubble.