You can make a bunch of money with something like Rails - if you're charging people directly, rather than low margin activities like advertising. See: Bingo Card Creator, Basecamp, and a bunch of other stuff. I'm a big Erlang fan, but objectively, Rails has a lot more to 1) get you up and running quickly and 2) iterate until you find some kind of product market fit.

Timely and relevant quote: https://twitter.com/patio11/status/587769019261829120