The "walled garden" is the online model of the old days of AOL and Compuserve, back when your ISP tried to run every service you would ever need, without any external interaction. That's not how we do it these days, because that model, rightly, failed. These days we have a different kind of walled garden, which I call the "roach motel". Content checks in, but doesn't check out.
Think of Facebook. It is possible to pull in anything from any other website to share on Facebook. Videos, pictures, articles, you name it. What you can't do, however, is take what you saw embedded on Facebook and share it elsewhere. That's what every big service online wants to be these days. The gravity well towards which everything online flows. All roads lead to Rome, hail Caesar Zuckerberg.
My concern is that it seems to be working so far. I can't see the catastrophic failure at the end of the rainbow, and that bothers me. One day, you see, Facebook will go away, one way or another. If it is where we kept everything, then everything will disappear.
Mokalus of Borg
PS - The only possible end I can imagine is a competitor just as big, stealing all the users.
PPS - That's not so much an end as a transition, and a very unlikely one.