That's the "problem" with global, real-time communications: people you don't like might use them, too. What I find more interesting are a couple of other points. First, there seem to be no open-source references to twittering terrorists. Are the terrorists clueless? Are they that good? Is nobody looking? Are the reports classified? Second, there is the ease with which the Army lumps all kinds of "hacktivists" with terrorism.
Twitter has also become a social activism tool for socialists, human rights groups, communists, vegetarians, anarchists, religious communities, atheists, political enthusiasts, hacktivists and others to communicate with each other and to send messages to broader audiences...Gee, is there anybody with a different opinion who we shouldn't be afraid of?
Twitter is already used by some members to post and/or support extremist ideologies and perspectives. Extremist and terrorist use of Twitter could evolve over time to reflect tactics that are already evolving in use by hacktivists and activists for surveillance. This could theoretically be combined with targeting.