Click Like, if you like to dislike:

Facebook dislike button

The easiest way to dislike something on Facebook

Just write or copy-paste this address to comments on Facebook:

You can test it on our Facebook page

Or if you don't want to copy-paste manually every time,
drag this link to your browser's bookmarks toolbar:
☹ I Don't Like
Now go to Facebook, click on a comment field, click "☹ I Don't Like" and enter.
See how to do it here.