According to Soundscan, the #1 selling digital song of 2010 was Katy Perry's "California Gurls." The tune sold a rather remarkable - by 2010 standards - 4,398,212 copies in the US, and over 8 million worldwide. In an era where almost no one sells any records anymore, this is pretty impressive.
As you may or may not know, I have something of an unhealthy obsession with Katy Perry. It has little to do with her music (of which I own none).