Question

Solution for my friends who sits 24 hours on Facebook.. let him know to study

Answer 1

Let him/her know about the causes he/she may suffer if he/she wouldn't study.
Let him/her see educational motivational videos on YouTube.
Or then also if he/she is not understanding,
then simply say your concern about your friend to his/her parents.
They will simply delete the application.
HOPE MY ANSWER WOULD BE HELPFUL TO YOU!!!!!!☺☺

Answer 2

First understand what he or she is doing on the Internet and facebook. Check if those activities are useful or not useful. Then ask him to avoid all those unnecessary things. 

Explain the importance of daily activities like playing sports, reading books on the study table, or talking to friends or family members.

Facebook is not the only important thing in life. There are other things to do which are important. If we do not do those other things, then we will become an introvert or not a good socialite.

Also always working on PC or mobile spoils one's psychology.

Pizzas, Chips and Sweets are good to eat. But if we eat too much regularly, then health problems arise. Same way, too much of facebook is bad for health and academic career.  Simple and effective explanation.

Show him good alternatives to spend time.