You're kidding, right? After they ditched ruby and wrote finagle, they've been super reliable. They've got one of the biggest Apache Mesos clusters in the world (second only to Apple's Siri backend per Apple employees who presented at MesosCon) and run it all with Apache Aurora, a really nice mesos framework for long running jobs.

Twitter, Soundcloud, Salesforce, Pinterest, FitBit, Tumblr, Box, Foursquare... What do they all have in common? They all run finagle in production. For java, it is about as battletested and as good as you'll get unless you want to write your own. Personally, I'm more of a golang and python developer, but respect where it is due. Twitter is massively more scalable than the overwhelmingly majority of the sites on the internet and they've gotten their act together since abandoning Ruby and the fail whale.