Show Me How To Play (SMHTP) is a music and tutorial site that teaches you how to play your favorite songs on real instruments. Playing Guitar Hero is actually a waste of time, as you actually do not learn how to play a real Gibson.
SMHTP is also offering multiplayer tutorials, so you can learn as a band as well.
Of course there is the whole social network and "show off your skills" feature set available on the site.
Right now they give away for free the Nirvana classic Smells Like Teen Spirit as multiplayer tutorial.
More details on the "Show Me How To Play" site.