I have to disagree with this comment, “Single player games are on the decline not because of lack of social interaction, but because in most of them, players ether get bored with them, or consume the content very fast and they just are not worth the retail price.”
I still play many of my single player games so do my friends. Civ is up to 120 hours, Fallout universe is more then 250 hours, S.T.A.L.K.E.R. - Shadow of Chernobyl is the same and the list goes on. The only time a game fails is if it is bad game, something like Robin Hood: The Legend of Sherwood, or lastly a game that misinterpret it's self as an FPS and is not - like Rage.
If they are well thought out games single players, if they are interested in the game, are loyal fans to the series. Most do not want to play online and will not buy a game if it is.