Within hours of Anders Behring Breivik being apprehended and identified, police, bloggers and journalists had found Facebook and Twitter profiles proclaiming his far-right ideology and commitment to taking some kind of action. Significantly, Breivik -- a fundamentalist Christian who lived on a farm in rural Norway -- appears to have created the accounts just days before the attack as a platform to explain his actions and guide people to his 1,500-page tract condemning multiculturalism and calling for revolution and civil war. Indeed, in a chilling statement which puts him high on the "psychopath"-meter, Breivik said his crimes were "marketing" for the text, which is your standard pseudo-intellectual craziness (apparently he plagiarized from Ted Kaczynski, a.k.a. the Unabomber, maybe to meet the 1,000+ page requirement for this kind of screed).
While less ambitious/pedantic, Jared Lee Loughner also posted a series of videos to YouTube attempting to explain (in psychotic fashion) his beliefs about language, mind control, and government oppression shortly before he shot 20 people in Tucson, Arizona on Jan. 8, killing six and wounding Arizona Congresswoman Gabrielle Giffords, among others. While these strange text-based videos weren't necessarily direct justifications for the attack, they provided a revealing window into Loughner's very disordered thinking, undermining arguments that he had what might be termed a "conventional" political motive (prison psychiatrists subsequently diagnosed him as schizophrenic and judged him unfit to stand trial).
Even in cases where the perpetrator doesn't leave behind a formal explanation, social media can at least give some hint about their mindset. For example when Nidal Malik Hasan went on a shooting spree at Fort Hood, Texas, that killed 13 people and wounded 29 on November 5, 2009, investigators were able to determine that the crime was ideologically motivated in part from Hasan's activity on Islamist Web sites.
But the move to social media seems to be a fairly recent development. When Seung-Hui Choi killed 32 people and himself at Virginia Tech on April 16, 2007, his explanations to the world, such as they were, came in the form of a traditional suicide note as well as a package containing photos, videos, and the requisite meandering text, which he mailed to NBC News to publicize to the world. More recently, when Steven Kazmierczak shot 26 people, killing six, at Northeastern Illinois University on February 14, 2008, police and bloggers turned up nothing online -- not even Facebook or MySpace profiles (true, the conspicuous absence of Facebook or Myspace pages could be attributed to an antisocial character, but that didn't stop Loughner and Breivik from logging on, lured by social media's ideal qualities as a platform for publicizing their views).