??? I think there are a lot of really popular applications that aren't written in PHP. In fact, excluding Facebook and Mediawiki, I can't think of anything I would classify as "popular" that is written in PHP. (Not sure whether to count PHPBB or not, seeing as although none of the sites hosted on it get a ton of traffic, cumulatively it must be one of the most popular pieces of web software ever)

Rails, specifically has Github, Twitter, Basecamp, and Discourse, just off the top of my head.