Whoever does: care required in talking to the media on this point. The New York Times article on this general area was really quite misleading, and has simply been copied in hundreds of places. The BBC is likely to be taken as a similarly "reliable" source. Anyone trying to compress the story to a Twitter-sized message risks misunderstanding. In any case the basic point I would always try to make is that if you take 2007 as a baseline, when many people were on enWP for reasons of fashion really, it looks like participation has dropped back quite a way; but once we had some proper statistics from 2009 it seemed clear that this year serious participation has been fairly constant.
Charles