According to DFC Intelligence, physical media is pretty much dead in the PC gaming industry. The market research firm told UK site PCR that downloadable formats made up 92% of worldwide PC game sales last year. That percentage is expected to keep climbing, as well.
Unfortunately, there isn't any information on how DFC Intelligence arrived at that figure. Most PC game downloads are sold through online services that don't report total sales figures. And most of those sales likely come from Steam, which is owned by Valve, a privately held company that doesn't publish regular financial reports. Getting an accurate bead on downloadable game numbers has got to be difficult.
Still, it seems plausible that the vast majority of PC games are sold in downloadable form. Thanks to frequent Steam sales, Humble Bundles, and other promotions, downloadable games are usually cheaper than their physical counterparts. New titles can be pre-loaded before they come out, allowing rabid fans to start playing pretty much the instant games hit store shelves. And, now that the big players all have their own download services, games are pretty much guaranteed to be available through at least one online outlet. I can't even remember the last time I bought one on physical media.