1. Inappropriate Pictures
It may go without saying, but prospective employers or clients don't want to see pictures of you chugging a bottle of wine or dressed up for a night at the bar. Beyond the pictures you wouldn't want your grandparents to see, seemingly innocent pictures of your personal life will likely not help to support the persona you want to present in your professional life.
2. Complaining About Your Current Job
You've no doubt done this at least once. It could be a full note about how much you hate your office, or how incompetent your boss is, or it could be as innocent as a status update about how your coworker always shows up late. While everyone complains about work sometimes, doing so in a public forum where it can be found by others is not the best career move. Though it may seem innocent, it's not the kind of impression that sits well with a potential boss.
3. Honesty is the Best Policy (as is consistency!)
If you say on your resume that your degree is from Harvard, but your Facebook profile says you went to UCLA, you're likely to be immediately cut from the interview list. Even if the conflict doesn't leave you looking better on your resume, disparities will make you look at worst like a liar, and at best careless.
4. Statuses You Wouldn't Want Your Boss to See
Everyone should know to avoid statuses like "Tom plans to call in sick tomorrow so he can get drunk on a Wednesday. Who cares that my big work project isn't done?" But you should also be aware of less flamboyant statuses like "Sarah is watching the gold medal hockey game online at her desk". Statuses that imply you are unreliable, deceitful, and basically anything that doesn't make you look as professional as you'd like, can seriously undermine your chances at landing that new job.
5. Not Understanding Your Security Settings
The security settings on Facebook have come a long way since the site started. It is now possible to customize lists of friends and decide what each list can and cannot see. However, many people do not fully understand these settings, or don't bother to check who has access to what. If you are going to use Facebook professionally, and even if you aren't, make sure you take the time to go through your privacy options. At the very least, your profile should be set so that people who are not your friend cannot see any of your pictures or information.
6 Losing by Association
You can't control what your friends post to your profile (although you can remove it once you see it), nor what they post to their own profiles or to those of mutual friends. If a potential client or employer sees those Friday night pictures your friend has tagged you in where he is falling down drunk, it reflects poorly on you, even if the picture of you is completely innocent. It's unfortunate, but we do judge others by the company they keep, at least to some extent. Take a look at everything connected to your profile, and keep an eye out for anything you wouldn't want to show your mother.