Since no one seems to be teaching the kids this anymore:
Don't post anything with specific registration numbers on it, kids! This includes driver's licenses (we're so proud you can drive now, but don't show us your name, face, address, and driver number!), passports (travel is fun, but just post pictures of what you saw after the trip is over!), train/plane/concert/show tickets (see above!), or credit cards oh my god people are posting their entire credit card online that will put you in massive debt and screw your credit score for life please do not!!!
This has been a PSA from your concerned friend online. Please protect yourself. Some secrets are not only okay, but necessary.