More publicity, which potentially means more customers, which means more money.
Also Facebook may give you a different customer base that you don't already have.
For example if you are selling fake flowers door to door, going to your neighbors, people at work or your family, or church etc, those are your only customers. If you start advertising in the paper you might get customers outside of that base of customers. If you advertise on Facebook, if only just your friends/family, then that can also add to your customer base. If you advertise using Facebook and joining groups or other pages, again you expand beyond your base extending your customer base.
However it also depends on how successful your advertisement is. Not all advertising means more customers. But you won't lose what you already have unless your advertisement is sooo outrageous that people get turned off, or your product is no longer needed or is not meeting the customer's needs anymore.