A Fellow of the American College of Surgeons is a Board Certified surgeon in a wide variety of subspecialties who is in good standing and who has applied to the ACS for membership, submitted case lists and been approved by their governiong body. I am proud to have been accepted into membership in the ACS and sometimes use the designation FACS after my name.
However, being FACS doesn't necessarily mean that you are an excellent plastic surgeon who will give the patients the best result they can get. So while it is nice to see, don't place it very high on the list of things you will use to screen one doctor from another.
Web reference: http://www.randcosmeticsurgery.com