It's fairly easy to construct a portfolio of web applications in whatever language you want to use and/or demonstrate competency in a language (eg contribute to open source), but it's much more difficult to say you worked on an application at the scale of Facebook. That experience is invaluable and the lessons you learn there are applicable even when you move on to applications in other languages.

You should also probably look at surveys of language adoption b/c PHP has much greater market share. Hacker News is a very skewed sample.