We don't, but a lot of people won't resist when one is being pushed on them. The facebook app just has to force enough "Install now!" Banners in their customers(/"products") daily view.

to prevent you from installing tons of individual apps? the only reason I can think off.. having said that, I don't use anything facebook related and don't like notifications that much.. I've overcame the fear of missing out years ago. I consume news when I want.