My guess is it is just a fad. Maybe she has seen a dog lick someone or has been licked by a dog and decided she either wanted to try it or it felt good to be licked. I would probably say just act like it is nothing until she starts licking your face, unless you don't mind. I believe it will pass. If it gets too bad, i.e. licking guest and friends, then she should be told it is not nice to do that to people she doesn't know.
I am not sure how long she has been licking things but if it continues I might ask my MD if she is ok. ORal fixations (Freudian Theory) should lesson easily by the time a child is 3 or 3 and a half. IN my own experience with children in this age group..putting things in the mouth stops before a child is 3 and a half. My son's Psychiatrist says that it could be a obsession/compulsion if she is anxious. Does she exhibit other fear behaviours or worries out side of the norm? BEsides the health issues it may become a peer issue too. good luck!