If someone here were truly releasing intelligence information, and the Government wanted to know the source, it would certainly not be real hard.
First, with issues of National Security, the Government could subpoena FR for the identity of the poster, provided the body of postings did not already reveal the poster's identity. (e.g. -- I am familiar with the process of such subpoenas to service providers in simple civil matters.)
In the case of Sean, who uses his full name and has his own cite, that would obviously be easy. That is, if he were putting out sensitive intelligence that the U.S. did not want out, for any reason, they would be talking to him to find the source. If, however, someone simply began rattling off intelligence in here, and that poster was truly anonymous, the Feds would move through FR.
Believe me, if you compromise National Security by posting critical information, they will find you. (Now, how they chose to get you to tell about the source of your info, that's another story. And, depending on the source, it may all get hushed-up.)
Most stuff we post here is harmless speculation and rumor. He says X, she says Y, what if this happened, etc. For the most part, this stuff is not not believed, but it has some propaganda/disinformation value that real spooks can appreciate.
You post real, hard data, photos, maps, movements, plans. You will find very quickly, that uncle Sam will be speaking to you. Of course, none of us here has such info, and even if we did, we would not print it.
The notion, however, that FR screen-name anonymity will protect someone is not true. To Sean's credit, he uses his full name, and he stands behind what he says -- even though I think he is way off. Frankly, Sean's stuff is what attracted me here to begin with. But, like everything else, you have to evaluate over time and compare with other info to assign credibility. Some folks think he is right on the money.