While it is commonly believed that the Internet has been a boon to Islamic terrorists, this has not been the case. The main reason for this is that the Internet gives terrorists the illusion that they have a safe, secure form of communication. But there are so many administrative tools available to police, that can detect this communication, that the net result is the Internet has become a prime counter-terrorist weapon.
There are techniques terrorists can use to make their communications more secure, but most don't know them, or don't bother to use them. Things like leaving email as a draft, rather than sending it, or using encryption. But even techniques like these make your messages vulnerable to interception. In the end, any use of the Internet can be intercepted. Often this is accomplished with commercial software and hardware designed for network administration, not spying.
The general public, and many journalists, are unaware of this situation. Terrorists tend to be better informed about the dangers of using the Internet, because so many of their cohorts have been taken down because their Internet communications were intercepted. But because Islamic terrorists tend to be rather too cocky, or too confident because they are on a mission from God, many continue to employ the Internet despite the obvious dangers.
One of the alleged great strengths of al Qaeda, after their Afghan bases were lost in 2001, was the dispersed quality of the organization. The problem with that is that most of these "dispersed" are untrained in the need for OPSEC (Operational Security, things like not using the Internet for critical communications.) The higher up the food chain you go, the less use of the Internet you encounter. At the very top, people like Osama bin Laden and his key lieutenants rely on human couriers, often to deliver memorized messages verbally. While the lower ranks of al Qaeda are entranced by the Internet, and other communications technology, the guys at the top are terrified of it. Mostly, it's a matter of experience. See enough of your chums get caught, or killed, because of cell phone, email or beeper use, and you get a bit paranoid of this stuff.
Often, the small fry are allowed to keep emailing and using their cell phones, just to monitor their "chatter" for useful bits of information. Out of many tiny pieces of data, often comes a picture of what the leaders are up to, and where they are. The Internet gives many terrorists the illusion that they are in touch, without realizing that the people at the other end have arrest warrants, not tickets to paradise.