How much FOD hazard can a Styrofoam drink cup pose?
Probably very little. There's the very slight chance that it could stuck on a pitot tube or some other terrible scenario in a Final Destination type film which, coupled with another issue, could cause a problem but again very unlikely.

The underlying hazard is the attitude towards FOD in general. It's a bad habit to build complacency towards any FOD sitting on a parking apron or letting it "get away". If it's SAFE to pickup the FOD then it should be picked up. It's possible the next time someone ignores a cup, it could have been used by a wayward mechanic to hold a sticky chemical that would ruin a pilot's windshield and his day if it  smacks and smears on the glass on takeoff. Again very unlikely but why risk it!