oh, that's a good point -- i was forgetting they can pair up 4 different paying customers. i think it must be rare though -- i see full ubers and full lyfts fairly often, but i've only ever seen 2 couples (& it's only $1 more for a pair).

Nope, I've had the unpleasant experience of a full pool a few times, they'll fill the back seat if they can.

But only if they are fully occupied > 90% of all time (which is IMHO not really feasible).