I find it very useful for photo-sharing and arranging things with big groups of friends. I coulnd't be without it personally.
As for whether it's ever going to make money though, I don't know. If they start charging it'll be the death of them, and if they start taking the piss with adverts due to shareholder pressure, it'll only take one entrepreneurial sod to setup a Facebook type jobby with an 'import all photos/contacts/messages' option to kill it off completely. Given that Zuckerberg has made more money than it's possible to spend, and that he still has a controlling stake in the company, he'll be more concerned with keeping it on top of its game rather than pandering to shareholders and slamming it full of adverts. For that reason, I can see its value dipping massively, but keeping its userbase.