Re: Captive Users
"On top of that FB has become the defacto authorization mechanism on the web. So many sites now require a Facebook account."
That's news to me; never seen such a thing. I think you're confusing "the sites I hang around" with "The Web".
FB is big so long as: a) People want to share photos and some other content online, AND b) have no easier method than uploading them to a website.
As it becomes less and less necessary to do "b" in the future as increasing peer-to-peer systems become built into phones (and probably clothes eventually) and ip6 makes it possible to associate access nodes with individuals FB will fade away.
Google provide a service that is multi-platform and until someone comes along with a better database and a decent way to search it (eg, ignore Wikipedia pages automatically) they're fairly safe. The key difference is that a database is a thing the challenger has to create by their own effort, whereas what is undermining FB is being created by the steady continuation of trends in computing that have been visible for 30 years and more.
At the current share price FB is grossly overvalued and that will never change; they have shot their bolt and made their fortunes so why would Zuk care anyway?
Google could be killed by a clever program (oh, happy day!), but at the moment there's no sign of that while the writing is already on the wall for FB.