With billions of pieces of content being shared on Facebook every month and bad actors constantly targeting the people who use Facebook, preventing spam isn't easy. Just as a community relies on its citizens to report crime, we rely on you to let us know when you encounter spam, which can be anything from a friend request sent by someone you don't know to a message that includes a link to a malicious website.
At Posterous we deal with our fair share of linkspam and blackhat SEO link-building scams.
We need a revolution here. Perhaps with Facebook's lead. They have probably one of the best ideas of what links and link farms are evil. I would love it if they opened up an Akismet-like service to identify spammy content or spammy links.
Or maybe we'll just build it ourselves. We'll see. But the user generated content world is threatened by a sea of bad actors trying to get their illicit pagerank or get that one eyeball to buy one more penis enlargement pill.