We talk a lot about scalability but what is it that we really mean when we refer to a system or service as scalable?
"A service is said to be scalable if an increase in system resources results in a proportional increase in performance."
To webscale computing increased performance typically means serving more units of work (pages, TPS) but it can also mean larger units of work (bigger datasets, many-where-clauses).
The main reason I like this definition of scalability is it separates the scaled from the scalable. I've seen plenty of really ugly systems go big - but vertically and at ludicrous capital costs. Yes, you managed to squeeze scale that thing but that doesn't automatically earn you the right to refer to it as scalable.
To me, scalability is an economic thing as much as it is a technical thing. You have to build wide and grow complex IP across a commodity platform - but if you can't maintain (reduce!!!) marginal cost while you're at it then you still haven't earned the right to call your system truly scalable.