At some point, I would imagine the distinction between capital and land becomes blurry, though. Economic rent can be had from either if the barrier to competition is high enough.

Domain names are a good example, because as skissane said, you could just make another DNS root. The trouble is convincing people (browsers) to use it. The problem in attempting to overturn Facebook isn't mainly the coding, either, but having a critical mass care. Those barriers don't seem like absolutes the way land is; they're just very high, high enough for those who control them to extract economic rent.