Why do you assume that you have to spy to get that info?
My parents would simply talk to me. They'd ask who I was hanging out with and where I'd be. Then they'd *trust* me. It's amazing how important that "T" word is. They made it a point to open the house to my friends... so they'd know who my friends were, what they were like, get to know them more than simply as that kid down the street. They set limits, and expected me to respect them, and trusted me enough not to go snooping around. Yeah, they didn't know everything that I did, but then you can't know everything your kid does no matter how much you spy. Only way to know everything they do is to lock them in a closet and never let them out.
Spying is bad because if your kid ever knows (and he will), you've effectively destroyed any relationship built on trust and respect. Your child now knows that you don't trust him. That you don't respect him. How anyone thinks that a kid can be brought up well in an environment like that... I just don't understand.
But hey, what do I know. It's not like I was ever a kid.