... to be fair, lumping Erlang (millions of production hours in the real world for decades) with Haskell and Scala is a bit silly.
Erlang is battle tested in the field since the late 80's. It just happened that the market (high-reliability, clusterable, multicore systems) came to it so it started to get hype.
1) I'm struggling to wrap my head around twitter being cited as an example of software best practices.
2) How many man-hours are we talking about here? How do we get that data?
3) Once such a thing exists i'll start believing the hype. I think you got that the wrong way around. The "mainstream" programming world has always been imperative (C and C like language). The "hype" is the other way round, Scala/Haskell/Erlang will save us from certain doom etc.