I was reading about vulnerabilities of onion routing and stumbled across correlation attack. From what I understand, it's possible for authorities to setup some effective analysis program, and de-anonymize most of the onion routing.
For example, I submit this question in 12:00 AM, the exit node sends it at 12:00 AM, the stackexchange server lists it as "questioned in Saturday - 12:00 AM". If I do this with multiple accounts, in multiple places, the authorities can gain confidence that it's my IP behind user Best Quality Vacuum.
What I want to know is: Has it ever happened severely, like spying effectively on most Tor users out there? Do criminals who're being constantly watched by the FBI get de-anonymized that way? Does one country have the power to do it, or does it need the permission from the rest?