Once the new friend clicks on a bad link, the spam begins propagating as his other friends do the same.

WSJ: Spam Finds a New Target

For instance, said Mr Hogan, filtering out e-mail messages that contain a web link can stop about 75% of spam.

BBC: PC hard drive, Eyewire

The weak link that has allowed robots to turn Webmail into a spam hose?

FORBES: Magazine Article

As Reddit notes, your news feed is more likely to be full of spam images if your friends are stupid unwise enough to fall for malicious link attacks.

FORBES: You May Not Want To Check Facebook At Work Today