Bernard Lunn's analysis of which businesses are viral, and why is a good read. One of the points he makes about communications applications is an especially good one:
Only Communication is a sure fire viral success. It only works when it is a genuinely new form of communication (webmail, social networking, microblogging). You cannot launch a web mail or social networking site today and expect viral adoption. But when it is a genuinely new form of communication, the viral adoption is stunning in its speed.
Bingo! That's the reason we chose to build our applications on the back of Facebook's API, rather than try to build yet another social network.