It's a little simplistic, sure, but I mean how much nuance do you expect me to fit into a HN comment? What I left out is Windows Mobile, which was Microsoft's attempt to extend the Windows hegemony to mobile devices. Here are Windows Mobile's US market share numbers, per Wikipedia:
2004 11%
2005 17%
2006 37%
2007 42%
2008 27%
2009 15%
2010 7%
2011 3%