After the like button was established on Facebook, many users wanted a dislike button to dislike certain status updates and other items. Because of the fear of the dislike button causing drama, and the diffculty of programming, it was never added as an official feature of Facebook.
The dislike button campaign took place from October to December 2009, with numerous groups and petitions for the implication of the dislike button. The Facebook team rejected the idea.
However, a plugin for Firefox and Google Chrome had created a Facebook dislike, and it was one of the more popular Firefox plugin, adding an virtual dislike button. Applications were also created to try to make a functioning dislike button.
Since there was no dislike button, the comment "dislike" was used to replace it, giving the same effect.
[Note that this information is not all true. Back in 2007 there were a Thumb up and Thumb Down button. People campaigned to get it back... I don't have specific date as to when it was implemented and remove, but it DID happened. Correct the page as you will.]