I would be interested in the number of licensed copies (including all releases and not just the current one).
My perception from articles, product compatibility, and other non-scientific methods, is that the most popular DAWs are (in no particular order) ProTools, Logic Pro, Cubase, and Sonar. I exclude Ableton and Reason only because they are conceptually different products.
Logic Pro, which by all accounts is an excellent product, has a bit of an advantage in that Apple practically gives it away so people will buy Apple Hardware. There is no other rational reason I know of for it's low price.
I don't like Pro Tools, I don't want to switch to Mac, and I don't know anything about Cubase (which has a steep learning curve), so the decision was easy for me. Also, I've been on some version of Cakewalk since it was a simple MIDI sequencer running on Windows 3.1.