Facebook does not have a dislike button at this time. There is, however, a dislike button out there that is a scam. Do not install a dislike button.
Don’t believe me? Check out some links on the topic:
According to Graham Cluley in a post in the Sophos blog,
“If you do give the app permission to run, it silently updates your Facebook status to promote the link that tricked you in the first place, thus spreading the message virally to your Facebook friends and online contacts.”
He goes on to say,
“But you still haven’t at this point been given a “Dislike” Facebook button, and the rogue application requires you to complete an online survey (which makes money for the scammers) before ultimately pointing you to a Firefox browser add-on for a Facebook dislike button developed by FaceMod.”
Be careful what you click in Facebook, it has scams just like the rest of the web.