until the world agreed that we'd use Facebook as a self-updating address bookAnd that's a valuable function, and I suspect probably the reason Facebook has seen the longevity it has.
It seems very vulnerable to disruption, though. Given a self-updated directory that others can use to find you if you change contact details, and maybe given also a single sign-on mechanism that you can use with arbitrary, separate services like photo sharing and gaming, what is Facebook's USP?
Their biggest barrier to competition has for a long time been the "everyone is on Facebook" networking effect. If it's only the occasional dissenter who isn't, then organising stuff and communicating using Facebook is easier than many alternatives, whatever Facebook does to their UI/UX/privacy policy. As soon as a significant fraction of people you actually care about aren't on Facebook any more -- and significant in this context doesn't need to be anything like the majority -- that compelling advantage just evaporates.
(Yes, I'm a Facebook dissenter as an individual, and a sceptic about the long-term viability of their business in its current form as an investor.)