That's possible, although i'd expect a company with Facebook's resources to be able to justify a move more easily than others, if PHP turned out to be a bad enough decision to really matter. It's still one of the names practically anyone would want on their resume, so they wouldn't exactly be unable to find developers in any language they chose.
You're severely underestimating how much lock-in there is to a platform once you choose it for a product and your entire company is built around that one product.
The amount of effort needed to rewrite Facebook is insane. To rebuild the entire site, migrate the data, ensure everything is seamless to the users, fire all the now-useless employees and replace the workforce (with all its own re-training costs)...
A company like Google can gradually phase out languages they use in favour of others because they can prototype things in new languages in other products they own (Think Dart and Go). A one-website shop just does not have that option.
Fair enough. I've never worked for a company that actually decided to switch, much less one close to the size of Facebook. My assumption was just assumption.
