there are a lot of rich people without cars (a lot of manhattan). i know tons of people who hate cabs and can't really justify a full-time driver--these people take a lot of ubers. also, cabs can only take 4 people at a time. the ability to call a large car when going out has a made a huge difference.
I know several people in the SF Area that take Uber almost everyday, multiple trips. It's not unreasonable to think that $/user in such cases is a few thousands.
