There's been plenty of media coverage about the fake Facebook 'Dislike' button.
It's a scam and is not from Facebook.
Do not use it. It will allow a rogue application access to your Facebook account and then send out spam.
"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," according to a blog post from Sophos' Graham Cluley. "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."