Just today, I saw a message from a friend that reads "Facebook just added the dislike button! Click On 'Activate Dislike Button' below to enable it on your account!".
If you are not aware of this, you may click on that activate dislike link which looks very legit but is a spam.
“There is no official dislike button provided by Facebook and there isn't ever likely to be,” Sophos said.
The scam managed to breach Facebook’s newly strengthened security walls to dump the come-on link on users’ pages, tucked in beside “Link” and “Comment.”
Clicking on the link, said Sophos, will forward fake messages to your online friends and run unwanted JavaScript on your computer.
If you are a facebook user, make sure you don't allow any unknown application any access to your facebook account.