Clarity is used to increase local contrast - which is the separation between tones on a smaller level.

In effect, it makes your image look more poppy/contrasty and i suppose "sharper".

The reason it looks worse on facebook is because the "higher level" or additional tones provided by adding clarity call for more visable compression, as apposed to your original image which probably has smoother tones and has less visible compression.

Sorry my explanation isn't perfect but perhaps you get the gist!

Good luck!

Clarity is used to increase local contrast - which is the separation between tones on a smaller level.

In effect, it makes your image look more poppy/contrasty and I suppose "sharper".

The reason it looks worse on facebook is because the "higher level" or additional tones provided by adding clarity call for more visible compression, as apposed to your original image which probably has smoother tones and has less visible compression.