I have to take issue with this assertion; I've seen a similar sentiment expressed here. Certainly, Erlang lets you naturally express concurrency in your application, but that doesn't automatically mean it will scale.

People have been writing high performance, message passing programs for a long time using libraries like MPI. The programming model used for creating distributed MPI applications is basically that of Erlang. The difference - and it is a big one - is that Erlang lets you do everything at a higher level with language constructs. This is a huge win.

But parallel applications don't automatically scale; scalability is a runtime behavior that depends on the problem you're solving and design decisions you made to solve that problem. How well a concurrent application will scale is usually not obvious.

There's no free lunch. You can't expect to start using Erlang and automatically get scalability. Like we've always done, you'll have to think carefully about issues such as data distribution (who has to know what when?), granularity (what's the smallest amount of work that should be parallel?) and synchronization overhead (does doing this concurrently actually cost more than sequentially?).