You can set the default sort order on category pages by going to Webshop Settings under the cog icon in the control panel. Then scroll down to the List Settings section and next to Default sort order for product listings select Most Popular from the dropdown menu. Save the change.
This setting will now apply to your webshop and Facebook shop. For more information on setting up your Facebook shop, login to your control panel and go to Content > Facebook App Editor.
Note: The theme design on your webshop can change the default sort of of products on the categories pages.