I'm sure you're aware of this idea the government is floating, whereby they will keep a database of people's friends on social networking sites - like Facebook. For more detail check out Shane Greer, who has a good post on the surveillance/big brother angle. However, I'd just like to point something else out to the idiots in the Home Office who came up this idea.
You see, even aside from the egregious invasion of privacy issues, this idea was flawed from the moment it hit the press. Because anyone stupid enough to join the 'North London Jihad' group or whatever, now won't do it because they know the government will be watching them. The government has just warned the dumbest fuckers around that they're going to be watching one of the places where they could get together and discuss their moronic ideas. Therefore, in the unlikely event that anyone was ever going to use Facebook for 'suspicious' reasons like this, they now won't and will be driven further underground where they'll be harder to catch.