I have yet to hear a compelling argument for why social network sites (or networking ones) should be used in the classroom. Those tools are primarily about socializing, with media and information sharing there to prop up the socialization process (much status is gained from knowing about the cool new thing). I haven't even heard of a good reason why social network site features should be used in the classroom. What is the value of knowing who is friends with who or creating a profile when you already know all of your classmates?
This not to say that technology doesn't belong in the classroom.... But why social network sites? To the degree that they support blogging and group sharing, sure... but that's not the key point of them at all. They key features that make them unique are: profiles plus visible, articulated and surfable friends' lists. I simply don't get why these are of value in the classroom.