A billion of people are using FACEBOOK.Users are expressing themselves and interacting with their peer and friends through wall posts, uploading their photos, passing information’s about events and other meaningful information and for that reason facebook needs a large scalable database.
I could imagine that is why it is a very popular Google search keyword.
I have searched a lot on this topic and come out at a conclusion that Facebook use several database techniques. The challenge for Facebook’s engineers has been to keep the site up and running smoothly in spite of handling close to billion active users.