BuzzFeed: Though we may not typically think of it that way, the creators of the MIT-based program Immersion are not wrong to call email “one of the original forms of social media.” Social media has become shorthand for the ever-expanding circle of more instantaneous forms of internet-based interaction — Twitter, Facebook, Tumblr, and so on. Email, without accompanying “like” or “favorite” buttons or a constantly refreshing stream, can feel, by comparison, slow.
If by most valuable you think of it as the most profitable, then yes. Email marketing is killing it.
As for social... I wouldn't call it that.