Yes, but that's a moot point unless one of the following is true:

1) Twitter monitors all tweets itself for any "subversive" content, and then takes the initiative to report that to the appropriate authorities.

2) Twitter gives full access to all tweets to the FBI, regardless of their content, so that the FBI can sift through the data themselves.

1 isn't such a big deal, but 2 certainly is.

I doubt Twitter gives them everything since it's a difficult engineering problem to handle that much data. But I could easily believe the FBI has a list of terms or phrases that Twitter self-monitors for.