How using Facebook at all can bring harm to you in real life.
Case in point: a colleague's teenage daughter applied for a summer job at the local supermarket, got selected and went to a job interview. The interviewer asked her to "describe herself". She did, and the interviewer then said "that's not what your Facebook says. Thank you, we'll call you." She's only 16 and she wasn't doing anything particularly public or outrageous on Facebook. It hit her hard and my colleague says she's been kind of depressed by the rejection she's faced.
As for me, I've learned the hard way 13 years ago (before Facebook but after Google) that the internet never forgets, and whatever you say will come back to haunt you eventually. As a result, I've learned to shut my trap online whenever possible.