Nothing is indispensable, especially so, those who believe they are!
I do not believe the view "it is this indispensable tool" I am pretty sure alternatives for every Faceboook feature exist! The one thing Facebook has been successful at, tapping into the human 'Tribal' nature that drives us to herd and want to fit in! - it is preferable to be at the table than on the menu!
But, indispensable, No!