Let me share some thoughts on it.
There are ~4000 tor relays. Users are not forced to be relays because the torproject thinks this is a dreadful idea.
Now, at the current moment it is assumed that two, possibly more, but for the sake of this paragraph two things can be discovered from an adversary passively monitoring tor traffic.
1.The fact that it is tor traffic.
2.The size of the tor traffic.
Now, take the size of a website like silk road and the "needle in a haystack" analogy.
Now luckily searching for a tor server is NOT like finding a needle in a haystack. That would be EASY, it is more like finding a very specific piece of hay in a haystack.
Now let's say someone is hosting a very large website within this haystack, now whoever is passively observing this piece of hay is noticing WOW this piece of hay is getting HUGE. In fact, it is much larger than any piece of hay I've ever seen.
Now this is when my hay in a haystack analogy sort of falls apart but bare with me...they say to themselves, "Hmmmm this server is producing quite a bit of encrypted tor traffic. I wonder what's inside. Oh woe is me I don't have the ability to decrypt 2048bit RSA and quantum computers won't exist for another 10-20 years!". "Why I know!", says the hog. "I'll Ddos the server, if this piece of hay disappears I'll know it's the one and I can eat the scrumptious peticular piece of hay!" or the hog says, "I'll hack the server" or any other vector of attack.
TL;DR
Tor and i2p should fuck and have a baby already half or more of these relays are probably compromised.