Everything done in the past looks easier once you see it done.
An analogy with Windows:
People don't give a shit about Windows. Or, really, their programs. What they care about are:
(1) their data (which requires their existing software, which requires Windows), and
(2) the ability to do something (which means software, which -- on PCs -- is essentially only written for Windows.
That, and that alone is what maintains Microsoft's dominance.
Think: how popular would Firefox, Thunderbird & LibreOffice be if they only ran on Linux? And if LibreOffice only understood it's own file formats?
Google Docs is breaking the lock that MS Office has on people, but only because it can read and write .doc and .xls files. The ability to manipulate your data anywhere is only useful if you can access the tons of documents that you've already created.
It's also the reason that "we" who see monoculture a threat push so hard for open standards.
Who knows what the next generation will prefer?
That's the pertinent question. My kids don't voluntarily use Facebook. But if they want to share something with adult ("hey, look at our vacation pics!"), they do.
Is there a Facebook API that allows users to access their pictures, tags, comment threads, everything posted on their walls, etc?
If so, then there's an opening for the competition. If not, it'll take a generation for Facebook to die, and a lot of memories will die with it.