Social networking sites such as Facebook universally support chatting facilities of some sort. They also provide email services or a message service of some sort that may resemble email for all practical purposes. Twitter is all about sending messages via tweets, mostly visible to any Twitter user.
Chapter 8 provides a thorough treatment of the issues involved in chatting and messaging, and the context of social networking doesn’t really change that treatment. So if you’re interested in more information about this topic, please see that chapter.