Do digital downloads of a "single" track count as a single or something separate?
If they do that would account (partially) for both the decline in album sales and the increase in single sales. As people now simply buy a large number of "singles" rather than the whole album.
A single. There is no longer a hard and fast divide between a single and a track from an album. The Rage single that got to number one was just a track from their album.