This is incorrect. You need more than a third of all Tor nodes to be able to have a decent probability of working out which user originated which requests. Just

This is incorrect. You need more than a third of all Tor nodes to be able to have a decent probability of working out which user originated which requests. Just running or observing the exit node is not enough (the exit node has no idea of who originated the traffic). Either that, or you have to be able to observe the traffic from most/all exit nodes as well as observe all the traffic coming from a user's Internet connection to be able to correlate with some probability what sites that user is visiting through Tor. Given any random person in any country can run a Tor node (exit or non-exit), both are quite difficult.