Isn't it rather trivial to find who is accessing a website if you can manage to monitor tor nodes? Just do some heuristic, to see when traffic happens, and over time, narrow down users.

If you're the FBI and have the authority to monitor the whole internet, isn't it trivial to catch any tor user?

Tor is still secure, but of course if you are the government and have skilled engineers, time and admin access to the internet infrastructure (by legit or covert means, I'm pretty sure the US can monitor traffic outside his jurisdiction), tor is not safe. But tor is still safe from countries other than the US, unless the US government have a problem with what you're doing.

I would still be curious to see if tor does counter this problem by passively sending traffic to avoid this. Anyway I stand that there are 2 kinds of security: security against small bad actors, and security against competent, resourceful, big actors. The latter is usually impossible to get because it becomes extremely fastidious and complicated.