I work for safeway aswell and i can tell you that at least 3 people have definitely been given the sack for comments they have made on facebook. The reason being that you are still representing the company. Admittedly these people were managers and as such are expected to have more respect and loyalty for the company.
I would say it will be up to the discretion of your store manager, if they like you and you're a good worker then you'll get a first and final warning, if they don't like you they'll show you the door. Out of interest what store do you work for?
Good luck