Heartbleed seems to be more plausible answer to how NSA has cracked Tor instead of the global whole-Internet-scale input-output analysis. Well, at least i hope it is so...
"If you actually look in to where these Tor nodes are hosted and how big they are, some of these nodes cost thousands of dollars each month just to host because they're using lots of bandwidth, they're heavy-duty servers and so on. Who would pay for this and be anonymous?"[1]
[1]http://en.wikipedia.org/wiki/Tor_(anonymity_network)#Exit_no...