It's not really just a site, it's a world. And the Facebook identity now extends beyond the walled garden. With Facebook Authentification, it's more like our online passport.
The world is pretty locked in.
I think there's a history there where certain social/discussion/sharing sites/systems are popular, for a while, they peak, then the next cooler thing comes along and the masses move on. Yes Facebook has hit the highest peak to date. But that doesn't mean it will last. When Zuck started it was the small cool elite site for Hardvardies. Now there's a lot of Moms and Uncle Bobs on there, the masses, dorks, rednecks, coworkers, bosses, and precisely because Facebook has violated user's trust a few times, and spread it's Auth system and Like buttons everywhere, a growing sense of distrust and avoidance towards it, precisely because you don't want Facebook knowing everything you do, or risk having the Moms and Uncle Bobs find out, coworkers, bosses, etc. Popularity is a double-edged sword: if you get it virally over just a few years you can lose it virally just as well. Now I do think Instagram was massively overvalued based purely on objective evidence to date. But I do understand how Zuck could fear it, based on future possibilities, and priced it based on fear. He knows how quickly Facebook's success came to him. He knows how quickly it might have come to Instagram. And arguably, past a billion or two in the bank, it's all just play money.
I can't remember any website that _required_ Facebook to log in, except maybe weekend hacks were other login methods were added later one. The online world may still have some freedom :)
FB auth is obviously a huge part of Facebook's extension beyond their walled garden, but there's plenty more examples. The fan-page widgets for social proof, Facebook Comments for publishers, and like-buttons are all areas of integration that solidify Facebook's lead.
It's not just a site, and it's growing into being more than just a platform. Already, Facebook's certainly beyond anything any historical precedents have set before.