Can't be too many other sites that let half the Western world post 35 photos of their Saturday night piss-up and charge them no money all for the privilege. Hardly a sustainable business model unless through advertising and/or the selling on of data. It's free to members for a very good reason.
There's kids born after Facebook was born whose entire life history of snapshots is held on Facebook, instead of in a family album somewhere. Such families rely almost entirely on Facebook for visual proof of their existence. Can quite see how Facebook need to turn a very serious profit to host...