1up reports that, in fact, PC titles only made up 14% of all game sales last year. As you might have guessed, Blizzard's World of Warcraft took the one and two spots on the list of top titles. Burning Crusade pushed some 2,250,000 copies in the US, while the base game sold about 914,000 units.
The article implies that one of the reasons for poor sales in the PC arena is piracy - a claim backed by statements from game companies going back years, and most recently explored by Call of Duty 4's Robert Bowling. The sheer number of people pirating the game took Infinity Ward's community manager off guard. These are comments from a man who worked on the #4 game on the top ten list, too.
The lesson is: (apparently) in order for a PC title to succeed in today's business market, you need to make a game that requires interaction with a central server. How long will it be before every FPS, RTS, and simulation we play on a PC 'calls home' every few minutes to make sure we're legit?