Haskell is the world's most popular lazy functional language. Simon Marlow provides the following information, useful for tracking its popularity.
It is possible to measure things like how many GHC or Hugs bundles are downloaded from haskell.org, but GHC and Hugs are distributed from many more places than haskell.org (for example they're included with many Linux distributions) and we don't have a way to count those downloads.
If the aim is to demonstrate popularity of Haskell relative to other languages, or popularity of Haskell today relative to Haskell in the past, then I often use other measures. For example, the activity on haskell-cafe:http://dir.gmane.org/gmane.comp.lang.haskell.cafeor the number of bugs reported against GHC:http://hackage.haskell.org/trac/ghc/wiki/Status/April07or the activity on the IRC channel:http://www.cse.unsw.edu.au/~dons/irc/