Tl;dr here is don’t take photos of boarding passes and post them online. Boarding passes are designed to be read visually (by those machines the staff wave them under at the gate), so, yanno. They actually have quite a lot of data on them.
(And, yes. You can print out dupes of someone else’s pass–they have little to no anti-counterfeiting–and use it instead of the person it actually belongs to.)