Is It Safe To Let Sites Access Your FB Account?

i was just at tagged.com and it lets you sign up with your fb account. you are then asked if you want to let it access your info. many sites do this, and i was wondering if this is a safe thing to allow?

or are you leaving yourself open to hacking???

a friend's fb account sent out xmas ecards last year without his permission! i told him to change his password. not sure how that happened.