I use Facebook
I was born in Earth
Did my schooling at Mars
Went to Saturn for higher studies
Got job in Jupiter
My hobbies are posting questions,answering , helping friends,programming
My friends are from Pluto
Facebook uses all your info's and help companies to place ads based on your interests and hobbies
For example :
If a company wants to pass a message on their next workshop to students from Saturn ,
Facebook will have user interest and details about students from Saturn and you have ads served specifically to them.
These ads are not free,publishers have to pay them !
Above strategy is only one of the several methods adopted by facebook.
Yes mainly they generate via ads.
Dont forget that facebook has acquired other companies like below to add up to their revenue :
But wait Whatsapp dont sell you ads !
Why we dont sell ads
Acquisition and profits at facebook make them easy to pay even ,
Software Engineer ,Avg salary / year = $1,11,864 per year
Approx 7476430.44 INR per year
6,23,035.87 INR / month / employee !