As to Java, sure, you can do anything with it, and do a decent job of it. But sometimes, as a startup, you are resource constrained, so if you can do more with less because you have a good grasp of a tool like Rails, or Erlang or Lua or whatever, that might make the difference. For a large company, Java is definitely a safe pick - no one ever got fired for choosing it: you'll be able to get whatever you need done, and "If we have to think a bit or throw some more kit at it, so be it." For a group like WhatsApp, Erlang seems to have been a good pick.
http://www.erlang.org/doc/efficiency_guide/binaryhandling.ht...
http://www.erlang.org/doc/programming_examples/bit_syntax.ht...
Erlang is really strong for that.
As to Java, sure, you can do anything with it, and do a decent job of it. But sometimes, as a startup, you are resource constrained, so if you can do more with less because you have a good grasp of a tool like Rails, or Erlang or Lua or whatever, that might make the difference. For a large company, Java is definitely a safe pick - no one ever got fired for choosing it: you'll be able to get whatever you need done, and "If we have to think a bit or throw some more kit at it, so be it." For a group like WhatsApp, Erlang seems to have been a good pick.