I friend anyone, if I know where they're from. i.e. if you're some random person and I have no idea how you found me, then I ignore you. But if you're from some group I'm in, or we've messaged a few times, etc., then I'll accept.
That being said, I have a privacy list for people I am personally friends with (not just people I've met, but people I would have over for tea). They have access to my information, everyone else just gets my hometown and current city, and my likes/interests. They don't know my relationships, my status updates, contact info, etc. My privacy settings are setup so that by default, you get nothing, and I have to specifically add you to my "Know Personally" list for you to see anything interesting.
I went through a games phase where I was adding anyone who played farmville, cafe world, or that fish tank game... then I got busy with school and decided I didn't want complete strangers whom I knew nothing about on my friends list, so I deleted anyone I didn't recognize.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker