If history is any indication, Facebook will fade out very slowly like Myspace, if there is a new competitor.
(Making the assumption that Facebook will be replaced this way is kind of stupid; Facebook is competent, MySpace wasn't.)
Say what you will about Facebook's policies, the company is run extremely well.
Google, World of Warcraft, Facebook, Windows... all of these things conquered their respective markets in such a massive way that every direct competitor is playing catch-up or giving up on reaching the same goals.
MySpace, like pre-Google search sites, was an obvious stepping stone on the path of social networking applications' evolution. It was terribly bad at almost everything and succeeded for a time purely due to demand for something of its general taxonomy.
Facebook, however, is a very thoughtfully realized effort that is largely an agreeable experience for users. As long as they continue to avoid introducing negative changes slowly and with minimal exposure to users, those users will continue to stick around and bring in others. [http://en.wikipedia.org/wiki/Boiling_frog]
And the big problem, one that I haven't seen addressed yet, is where people should go when they leave Facebook. There is no comparably-featured competitor, nor one even close. And of course, social networking applications are worthless without the people you want to interact with using them with you.
Everything eventually comes to an end; Google, Facebook, etc will be no different. It's just a matter of time.
They have been stuck with a low-end image for a very long time and will not be able to take back the lost momentum from Facebook. Add this to the fact that it has been bought out by Murdock, which means there is very little incentive money-wise for those employees to put in the same amount of effort that went into pre-sale Myspace and Facebook.
Just because the previous top social network eventually faded away doesn't mean we should expect the current one to do the same in any similar manner or time frame. There's quite a lot that differentiates today's Facebook and the MySpace of old. For one thing, how many third parties ever built on MySpace as an application platform, to the extent that they are today with Farmville and Microsoft Docs and the like?
I agree with your comment in general, but that statement isn't true. Apple are still a niche player, except maybe in America.