Right, but you can't just have two independent websites when people expect to be able to connect to all their friends. And with software being such a young field, scaling up is not always like just building a bigger factory - sometimes serving 10x as many users as you used to is something that no-one has ever done before with your kind of workload.
Luckily there are enough software people like yours truly who know how to solve these problems. Solving them requires tradeoffs, so you also need product people who understand those tradeoffs and make the right calls. This is what failed at Friendster, and not at Facebook or LinkedIn. Read this:
So that sounds to me like "you can keep your per-user costs linear as long as you're willing to sacrifice small parts of the user experience as you grow". Which is fair (though even then, making these tradeoffs is work).
Obviously if you're profitable per-user then scaling is a nice problem to have. But I struggle to see how scaling from their current users to say 10x could move twitter or quora from unprofitable to profitable; I think the engineering costs of making these tradeoffs and solving the scaling problems would largely outweigh any economies of scale, and I don't see how more users would increase their per-user revenue.
