I don't operate a serious web facing business.  Actually most successful webshops start designing their  custom code as they grow.

facebook.com operated on php/js/mysql LAMP stack until it grew into the multi-million users mark.  I believe their back end is now  C++, Java, Python, and Erlang.

Mt. Gox does pretty well on php/mysql

There are dozens of other examples. 

I just don't know Rails.  Here is someone's list of top 50 rails sites.
http://goo.gl/ekrGA

Seems a little light on  financial service site.

But like I said, It not the language as much as it is the dev's knowledge of that language.

So to each his own.