I don't think Yahoo mail or any of the other big mail providers find it necessary to assert ownership rights over content...
I'm no lawyer, but my guess is that email has relatively well-established case law. Email is old tech, and it's substantially similar to paper mail, which is really old tech, older than US law itself.
Whereas there's not a lot of case law about, say, a Facebook app which allows you to superimpose LOLcat-style captions on your friends' photos. Or a Facebook app (which I've just thought up, but which may also already exist) that allows you to ask your friends to sing phrases into their computer's microphone, then helps you construct a techno dance tune out of those phrases. Or any of the hundreds of other Facebook apps that encourage you to use your friends' data in novel ways.
