you can suggest this way
friend don't you think these days you are spending a little too much time on facebook. It's all right if you do your work properly but what I see is that you aren't doing that as well. Have you noticed how your score has degraded in the class. Three days in a row you haven't completed your homework. Dear please listen to me and stop this stupid addiction to facebook This is for your own good. Try to understand.However I'm sure you'll know the right thing very soon and regain your performance once again.