While Uber may be changing things, most of the literature that I am aware of in this area suggests that regulation of taxi services is necessary to get near a social optimum. It's also interesting to note that not all kinds of regulation are equally good -- e.g. some argue that controlling the number of medallions is less effective than controlling fares. This paper provides a good overview, I think:

http://www.sciencedirect.com/science/article/pii/S0191261501...

Unfortunately, I can't find a free version -- sorry!