I go to a lot of art & music performances. Lately, and particularly for the last few events I’ve attended, the performer emphasized joining their (MailChimp-managed) mailing list, because they didn’t want to seem uncool by promoting Facebook.
I don’t know exactly what it looks like, but I strongly suspect the social graph that disrupts Facebook gets built on email, somehow. Something like MailChimp, but without the direct marketing/conventional newsletter expectation baggage and a focus on message consumers as well as producers.
The advantages of “push” style communication, flexibility in styling messages, and potential for advertising all seem to me like a revolution waiting to happen. The openness of email, combined with the fact that everyone has one (even millennials!) are icing.