An AP perspective follows below. I think the many-to-many communication channel (email, SMS, and IM, initially) generalization dimension is interesting.
The first Internet e-mail system arrived in the early 1970s, and it has been an integral part of most people’s lives for at least two decades. Though e-mail is still a primary form of communication for older adults, recent studies suggest this is not the case for young people. Text messaging has surpassed face-to-face contact, e-mail, phone calls, and instant messaging as the primary form of communication for US teens, according to a 2009 survey from the Pew Internet and American Life Project.
Facebook sees its messaging service as a way to deepen its connection with the more than 500 million users of its network. If it can persuade its vast audience to become faithful users of its messaging service, Facebook conceivably will have more opportunities to sell advertising that caters to their likes.