Start-ups use non-Java for their main apps. But the other "infrastructure" stuffs that they developed after 5 years of existence usually required Java somewhere

Start-ups use non-Java for their main apps. But the other "infrastructure" stuffs that they developed after 5 years of existence usually required Java somewhere. Hadoop, HBase, Hive Cassandra, JMS are all Java solutions. Maybe it's for performance, maybe just because Java has more libraries for that particular domain. Google (pre-dominantly Java), Amazon (Java shop), ITA (they also use Java EE to my knowledge), Facebook (HBase, Hadoop, Hive, and maybe even haystack?), Zynga, Flickr (small part of back-end), Twitter (back-end stuffs), LinkedIN (Java shop), Last.FM (back-end stuffs), PowerSet (they came up with Hadoop)? I don't know why Twitter uses Scala for some part of their systems when eventually they need to write some Java code (and probably make that Scala code talk to Java infrastructure). If you're not writing bajjilion code for that particular portion of your systems, I don't see why you need to have both. Just pick one. Perhaps they have other reasons (personal or not).