While Facebook uses PHP where Twitter uses Rails, Facebook uses a plethora of languages to make the whole system work. So Twitter really isn't going to Scala any more than Facebook is going to Erlang. Which is the say that they use the best tool for the job, not one tool for every job.
To be fair, the Java ecosystem made the error of recommending the Java language and platform as the right tool for every job in the past. Today, Java infrastructure providers have shifted to offer a broader set of tools to help right-size the infrastructure to the project needs. This will only continue. And as it does, PHP, Groovy, Ruby, Java, and several other languages will be used together to help customers drive business results. The customers I speak with aren't backing away from their Java investments when they ask for broader dynamic scripting language support within their enterprise application server. Rather, they see value in using different language environments to extend the value of their Java investments. Maybe I'm biased, but Java the language and Java the platform have little to fear from PHP or other scripting language environments.
What do you think?
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p.s.: I should state: "The postings on this site are my own and don't necessarily represent IBM's positions, strategies, or opinions."