https://www.eff.org/pages/tor-and-https
That's how the Harvard bomb threat guy got busted, because his Tor usage was a big flashing red light among non-Tor users.
http://www.washingtonpost.com/blogs/the-switch/wp/2013/12/18...
That's how law enforcement knows what haystacks to search.
And since the OP used his own IP address (didn't go to a coffee shop), that narrows the search by 3 or 4 orders of magnitude.
Does anyone have a reasonable estimate on what per cent of Tor entry nodes are known to ISP's, law enforcement, and the NSA?
Tor is designed to hide who you are talking to, not the fact that you are using Tor. IPs and other metadata about normal Tor relays are published publicly by the network and are used by the client to build circuits through the network.
There is a special form of hidden entry node called a bridge that is designed for use in censorship-happy countries like China, but using them is a manual process that isn't the default. Traffic through bridges is a very low percentage of overall Tor usage.