In recent years it seems that functional programming languages have had a real popularity increase. Languages like Erlang, Haskell, Scala, F# and Clojure seem to be pretty well known and many popular programming sites (such as Stack Overflow) seem to be full of questions and discussions on them.
The question is how would you go about researching the growth of these languages in real terms. Of course I know about the TIOBE index but is that really the best source to measure the popularity of a given programming language?
Hopefully this question will be allowed since it is something that many programmers look towards to decide whether a given technology is likely to last for a long time or not (although this is not my personal goal).