Based on Marc's reasoning given on the site, sounds like they have an easy pivot into annotating everything if the rights issue takes them out of the lyrics game...
http://techcrunch.com/2011/11/10/google-buys-contextual-rich...
Guess what? It's now discontinued: http://www.google.com/sidewiki/ It's not trivial to build annotation platforms that are both compelling and easy to use. Hat tip to RapGenius!