According to the revelation by Electronic Frontier Foundation last week, an FBI or Federal Agent can make friendship with you at Twitter or Facebook to gather more information about crime investigations. The report further described that fed agents pose as "friends" and get connected with ordinary citizens on Facebook and other sites in order to gather background information for crime investigation. This will help them find out suspects on the run.
Now, the question arises about the legality of this type of activities. As per AP news stories, the middle aged lady Lori Drew posed as a teen boy on MySpace and harassed a 13-year-old neighborhood girl who later committed suicide. She was arrested for violating MySpace's terms of service to pose as someone that she is not and convicted in L.A. before a higher-court judge overturned the ruling. Fed prosecutors are making similar type of violation. Facebook has terms and conditions that makes it illegal if the user gives false information while signing in for the account.
Former U.S. cybersecurity prosecutor Marc Zwillinger argued that federal prosecutor can go to cyberworld as they do on the street. Fed Prosecutor claimed that they have been able to track down fugitive successfully after making friendship with his friends on the social networking site.
source : http://blog.taragana.com/index.php/archive/your-next-facebook-friend-may-be-an-fbi-agent/