They're not advertising that 2 million songs have been sold, they're saying that there are 2 million songs available to purchase on iTunes. There are FAR less videos (in the hundreds, thousands at the very most).
iTunes has sold over a billion songs, and 10 million or more videos.
As an aside, I find it funny that Yahoo! has ads for iTunes, when they operate a competing service. I've also seen ads for Tiger, specifically advertising widgets on Yahoo!, and they now own Konfabulator.