For those of you designing cover images for Facebook, I have created this image in order to aid your design. You can open it in Photoshop (or GIMP or whatever your design program of choice is), add it as a layer, and then add your content over it. Keep in mind that this template is for Business Facebook pages only, personal pages use a different template. Please see this post for the Personal Page template.
The overall Facebook cover image size is 820 x 360. However not all of that should be used for vital information, as the top and bottom portions are only visible on mobile, while the left and right portions are only visible on desktop.
This template is at 2X scale, so it's 1640 x 720. The reason for that is that you should be designing your cover image at twice-the-size for users with High-DPI displays (most mobile users), even if you don't have one yourself.
Here is what the template looks like (skip to the bottom of the page if you just want the download link, this is a reduced-size version):
When installed as your cover image, here is what a user on a Desktop computer will see:
Meanwhile, here is what a user on a Mobile device will see:
As you can see, you should put your most crucial information in the red.