The use of colour.
The homepage colours are pretty simple being mostly white and blue but because of the simple use of the colour blue it has made the association with the colour to be Facebook, when people think of blue the now think of the sky, the ocean and facebook etc.
The white in the homepage is mainly the place to put your details in if its your log in details so sign back in or your personal details to sign up making it white has made it easier for the viewer of the website to easily understand what its there for. Blue is a pretty standout and frequently used colour within Facebook, most of the writing has been done with blue except that actual title 'Facebook'.
