Of course, at the end of October last year, Just Dance 2 had only started selling. Since then, it's been doing very well for itself, and has been followed by several spinoffs, including Just Dance Kids (and its sequel!), Michael Jackson: The Experience, The Black Eyed Peas Experience, Just Dance 2 Summer Party, and, of course, Just Dance 3.
Too damn many Just Dance games can't claim all the credit for the growth in dancing-machine sales. Dance Central is at somewhere over 2.5 million units, and it's been followed by a sequel; Zumba Fitness has been a phenomenon, and Sony's Everybody Dance is also available.
Update: corrected Dance Central sales figures.