When uploading photos to Facebook they are automatically resized to match the Facebook website. Thats pretty handy but the algorithm Facebook uses can decrease your picture quality. The difference can be tremendous if you look at the image above.
The guys from fstoppers.com have a goos solution to this problem.
In short: Make sure that the long side of your image is 2048px or if you want to have a smaller picture 960px.
For the full story and how to export your pictures in those dimensions have a look at their post at http://fstoppers.com/how-to-size-your-images-so-they-show-their-best-on-facebook