There is a treasure trove of personal information wandering around MySpace. It is generally a young, vibrant base. The NSA might think it is a great site to do surveillance on, figuring that with the sheer numbers there must be someone dark and dangerous hiding within. I think they might find a few sick folks, but I doubt they are going to be plotting actions contrary to national security. MySpace gets way to much attention for that.
But the Pentagon could have a use for it – tracking down draft dodgers, should the need ever arise.
***UPDATE***
On the NSA combing through MySpace, Bruce Schneier says no surprise.