I'm all for our law enforcement and military agencies having all the tools they need to protect us from criminals and foreign threats. Data mining is one such tool, and ultimately I don't find it any different than any other form of investigation. It's just faster.
But, there do need to be the same checks and balances in place to make sure that any information that's mined is used for appropriate purposes. And frankly, I don't know if these checks and balances are in place. This story doesn't say.
Probably worth some investigation.