Are you kidding, Fred? It's more like these plazas you see around Manhattan which have a little plate set in the concrete saying that "Easement is revocable at will" or some such -- specifically denying that any "right" was created just because they put up a fountain and trees and a few benches.
Jimbo doesn't have to keep this site going. And WikiMedia CAN suddenly shut it down or go commercial or (shudder) pro-Marxist at any time...
There's no contract. No rights. You and me and mav and thousands of others have only 1 right: the right to fork. But we are all LITERALLY "allowed" to post at Jimbo's whim.
Gradually the committees are putting Jimbo out of the "whim business" so there's a bit of transition going on. But I can shut down the whole thing anytime with 2 mouse-clicks, and I wouldn't be violating your rights in any way.
Firmly (but fondly),
Uncle Ed