http://rzuwtppveur6yh3n2in7trmddgrkuhww4eb2znykgjwubihfpljiyuid.onion
If you select new entry and exit relays each time you use the network, the attacker will be able to correlate all traffic you send with probability around (c/n)^2.
But profiling is, for most users, as bad as being traced all the time: they want to do something often without an attacker noticing, and the attacker noticing once is as bad as the attacker noticing more often.