Looking at Sophos' report on the Facebook "Dislike" button, which has reportedly spread virally through the social network, I noticed a pattern consistent with several other scams. Recognize these red flags, and you won't be fooled again.
Now that you're skeptical, take a look at the Facebook status update in question. At the bottom, you'll see time elapsed since post was written and how it was delivered. So if a post comes from the web, it says "via Facebook." and if it's an update on someone's Mafia Wars progress, it says "via Mafia Wars Game."
Finally, some apps will give themselves away by sending you away from Facebook to an external website. The "Dislike" scam, for instance, would send people to sites such as fbdislikeit.info. This is the biggest giveaway of them all. Any time you're sent to an external website to install something on Facebook, use extreme caution and never install anything from a source you don't trust -- especially when the source claims to be "official.
The good news about this particular group of scams is that they aren't that harmful, and you can undo the damage by removing the application from Facebook's "Application Settings" page. But at that point, everyone will know you've been had.
See also:
5 Facebook Schemes that Threaten Your Privacy
8 Crazy E-Mail Hoaxes Millions Have Fallen For