The packaged retail sales for PC were 2.3 million, but Skyrim has sold a hell of a lot of digital downloads on top of that. It was described by Valve's head of business development as "the fastest selling game in Steam's history." And it has consistently remained in the top 5 or top 10 sellling games on Steam for the past nine months, which is quite remarkable.
The digital sales numbers have never been announced, but I would not be surprised if it sold at least as many downloads as packaged retail PC copies, if not more. That would put the total PC sales higher than the total PS3 sales, which were less than 4 million.
This is not true. Skyrim sold 2.3 million packaged retail PC copies. The total digital download sales have never been announced by Bethesda, and Valve does not release this data to market research firms like NPD, Nielsen, etc.