Yes, I think I am wrong to use the term walled garden, but it's hard to think of something else.
In a "platonic ideal" of the internet everyone would have their own internet connection, and a server and say post their own interesting queries and somehow others would find and answer them.
Perhaps search was assumed to solve it all then.
But the universe is much more "clumpy" than that so people will gather around certain locations, in nature they are natural oasis.
Perhaps we should drop the walled garden idea - gardens, walled or otherwise need tending and upkeep and that passed the ability of one or two people to do in their spare time somewhere around 1991 on usenet.
Tending a garden is a costly affair.
I think perhaps walled city might be a better term? It implies the "never leaving" which is what facebook seems to aspire to.
perhaps a better analogy is "chargeable car parking". :-)