According to research by ExactTarget, the answer is still email. It's more effective than Facebook and Twitter, but I'm not sure if that's just a matter of time as society experiences a massive paradigm shift in messaging/communication.
Email was a major paradigm shift from phone, fax, and mail. Messaging apps, with its multitude of features, is becoming a major form of communication. How often do you use Whatsapp, Line, WeChat, Skype, etc versus email? Once you can do everything on your messaging app, then email will become less relevant. We're still a long way from there, but it's coming.