Walled gardens can last a long time. You could argue that if a walled garden lasts as long as a technology is relevant, before open source competitors can come in, the walled garden has 'won'. I'd argue in this sense, Windows won: it was never bested by Linux on the desktop, even though it is now largely irrelevant.
It could turn out that Facebook stays prosperous until the world shifts to the next big thing, but well before any open source alternative takes hold and dominates.
You're missing the fact that Windows, as far as walled gardens and open standards go, is somewhere in the middle.
I.e. to run your applications on Windows, you don't need approval from Microsoft. To install Windows on a PC you also don't need approval from Microsoft. And the SDK has always been free of charge (contrary to other alternatives at the time).
I'd say it's a bit early to call Windows irrelevant - certainly one look at Microsoft's bottom line would suggest that Windows is still very relevant.
In some ways, yes, the choice of operating system is now largely irrelevant - if you mainly use web based stuff. But then again, walk into any corporate office in the world and there is a massive, massive probability that the only operating system you will see on desktops is Windows.
And also a massive probability that everyone there is running Office, plus a bunch of internal applications written specifically for that office, which only work on Windows.
His point still stands somewhat. Windows won by maintaining control of the OS market, though the OS market became less directly relevant due to the fact that the web became the dominant force in commercial computing.
That irrelevance is exactly what I'm referring to. Sometimes it doesn't come in the form of a direct competitor. Sometimes, open protocols route around walled gardens.
It may not be a Facebook clone built in a distributed fashion that forces Facebook to open up, it may instead be a whole new approach to social applications that nobody's thought of yet. That's fine.
It could turn out that Facebook stays prosperous until the world shifts to the next big thing, but well before any open source alternative takes hold and dominates.