I haven't used Tor or read much about how it works, but I'd heard it's rather poorly designed and has had a few security issues in the past. If that's the case, it's more an example of a system with design flaws that got used against it than a successful blocking of P2P in general.
It's not poorly designed at all. Tor is very well thought through by some very smart cryptology experts. There are some possible attack angles, but they usually have to do with 3rd party programs revealing your IP address (for example, a Java applet in your web browser), not design flaws in the Tor system itself.
Other attacks are completely academic and require control over the entire internet to work.
There is a lot of panic mongering and mud-throwing around Tor, but of the P2P anonimization programs it is still the most proven of the bunch.
ToR just f8ing works.