I attended (photographed) a conference yesterday on Marketing in the Digital World and I learned (or confirmed, with facts and statistics, things which were intuitively logical to me) a thing or two.
Facebook is replaceable; as a social medium, it has outlived others such as GeoCities, MySpace. It's longevity has to do with the emotional cost of moving to another media. Whereas moving to another email service merely involves informing everyone on your contact list of your new email address, moving from Facebook involves, potentially, breaking off "friendships".
In today's digital market, where supermarket customers can scan QR codes on the spot for price/quality comparisons, the customer has become the "deer with guns in the woods".