Before the days of social networking, hackers and malicious types had to rely on email to distribute their evilness. They'd send an email with an infected attachment of something that looks interesting that are designed to tempt you in opening the file. Things like "Cat does handstand on top of dog's head", "Naked Man with big tennis balls at Wimbledon" or "Cute girl on Channel 4 Countdown spells rude word" - most people caught onto the fact that a lot of these unsolicited emails will most likely contain a virus meaning that the trust in email attachments has been lost and therefore the possibilities of attack via email becomes much less.
Moving into the 21st century and the overwhelmingly popularity of social networking sites, such as facebook and twitter, the malicious hacker is provided a whole new and far more powerful means of infecting hundreds, thousands or even millions of people - the reason is TRUST.
