its not just resource constrained environments, but use cases where the advantages of a c++ web app outweigh the disadvantages. C++ web apps are fast. They are low memory usage (especially compared to solutions like java/RoR or PHP). They tend to use little cpu. Ultimately, once a little ground work is done, you are still designing the interface in html, javascript and css, and have all the advantages and speed of development of that delivery method.

For some definition of the word, I believe that Facebook is effectively deploying a c++ web app as part of its solution at this point.

There are many use cases for which RoR or PHP is the better solution, but it turns out it is imply true that there are others for which a c++ web app is the better solution.

Go figure :)