The suggestions are intended for those sharing on social-media sites through personal accounts, not for those who share for business reasons. However, even those running corporate accounts might find some of these tips useful.
Don't share pictures of people unless you've asked them if it's OK or absolutely know they're fine with it. It's only polite, especially when you're not in a public setting.
Never share pictures of children, unless their parents have said it's OK and if the kids themselves are old enough to say they're fine with it (I'm especially thinking about teenagers here; they might like to be asked). Related: don't post the names of children unless you've asked.
Don't check in to your children's school, unless you really want the world to know where your kids go to school. Do you? If you're sharing a picture from a school event, you don't have to have a location attached to it.
Don't check in to your house, unless you want your home address potentially known to the world.
Don't check in to other people's homes without asking them.
Don't check other people in to events or places without asking them.
Don't tag people in photos, unless they've given permission or you're certain they won't have a problem with it. Yes, Google and Facebook would love you to do this and may even prompt you to do so. You don't work for them.
Don't post pictures of your credit or debit cards. Seriously, people do this!
Don't post your phone number, unless you want it potentially exposed to the world.
Don't share or post things that may get you in trouble with your employer. Know what your workplace social-media policies are. Don't assume that what you say and do on your personal account won't somehow get held against you at your current or future job. It all goes on your permanent (social media) record.
Don't share that private post from someone else with the public, without first asking for permission.
Don't assume that anything you share privately will stay private, regardless of your own privacy settings. Once you post, assume it will be public. See also: the point above. Some of your friends won't think to ask.
Be respectful when commenting. Here's a rule of thumb: Comment as if you were talking to the person face-to-face. It's astounding how rude and unnecessarily mean people will be online. We'd never tolerate such behavior in person. We shouldn't tolerate it simply because it happens online.
Don't use social media when you're out being social in real life. This is a rule I violate all the time! But when I was unplugged recently, I found that it was kind of a relief not to think I needed to share that picture of something cool. I could just, you know, look at it myself and enjoy it more in the moment. There's always time to share in the future