Everyone working in the social media space looks for viral effects of the messages they tap into the channel. Not very many succeed, and if they do the format needs to be really creative, crazy or dead revelant to people in the space where the message is launched.
What you really would like, though, is what Mich Joel refers to as a viral expansion loop. This is when the message or service that is promoted creates greater value for the user if more people are using the service. A great example of this is Tripit (that can be used on LinkedIn). With Tripit you can see if a person in your network will be travelling to the same destination as you at the same date. This is great if you would like to schedule in a meeting, or just an occational beer, while killing time at the airport. The point is, if you get all your contacts to use the application it will bring you greater value.
If business people and marketers spent some time thinking of what service/ application they could bring to market to obtain these effects, their messages would spread naturally and have a much greater impact!