There are a couple of things we need to note on this one:
Firstly, you can't really limit a communcations channel at all. This particularly goes for electronic communications. As long as some form of networking exists, you can hide information in data transfer in countless ways (a famous eg. is
steganography), and the people who are often the most clever in coming up with new ways to talk are the people that really need to say something. The alternative is to cut off internet communcation, but do that and you'll have an army of internet-addiction-withdrawn preteens quickly mobilized into an army. Not to mention the disruption to business.
Second, because you can't limit a communications channel, you can't limit free speech either, because people can always use covert means of communication to say whatever they want. The people whose speech ends up getting restricted are generally people whose speech you don't need to restrict, ie. the law-abiding citizens. Not to mention that this breeds a growing distrust of government, which leads to Very Bad Things.
The best alternative, in my view, is to encourage free expression, not limit it. Open expression allows people to get ideas out in the open and can allow society to moderate its own discussion. In the same mindset, the best defense against terrorism is to know and understand your neighbors, because this way you build a web of trust and a close-knit community. Then you don't have to pose questions like these.