Facebook – maximum picture size
If you are uploading pictures to Facebook, the largest dimension in width is 2048 pixels. If you upload a picture with a width or height larger than that, Facebook will resize it, maintaining the aspect ratio, but you won’t be controlling the compression or other settings.
If you want the best quality photo, load your photo into Photoshop, make the largest dimension 2048 pixels, save it, and then upload it to Facebook.
Here’s a link from Facebook for more info.