There's little doubt the release of Fallout 4 was the gaming launch of the month -- and arguably the year -- and the anticipation has been reflected in the numbers. According to developer Bethesda, the first person RPG generated "record sales" across all three major platforms.
Now, it's easy to misinterpret the numbers when companies announce these sorts of things. For instance, the press release states that "approximately 12 million units" were launched "worldwide", adding up to "sales in excess of $US750 million". The wording however makes it clear these are retail boxes, despite muddling the issue a bit by mentioning digital Xbox One sales in the same paragraph.
Before you ask, yes, Steam is talked about in the very next paragraph -- it's the "number one game played", with close to half a million concurrent users -- a record for the platform. While there's no specific values forthcoming from Bethesda, SteamSpy provides a rough count of 1.6 million owners, give or take 30,000 or so.
Anywho, it's sufficient to say the game has sold well, despite negative comments about graphics and frame rate before release. Seems we can't get enough of the wasteland.