Your friend posts on facebook that she wants you to be a part of her 'Secret Sister' gift exchange- and it sounds fun! You buy one $10 gift, send it to one of the 'secret sisters' on the list... and receive dozens of gifts from other sisters in return!
The problem; it's not just impossible, it's illegal
Let's start with the impossible part- this is a pyramid scheme. If everyone is only buying one gift, it is impossible for everyone to get multiple gifts. You can't make something out of nothing. Someone at the bottom of that list is going to get scammed. And it will probably be you.
That brings us to the illegal part. Ponying up $10 in exchange for an unknown number of gifts is considered gambling, and gambling using the postal system is against the law. The BBB outlines it really well:
The U.S. Postal Inspection Services says that gift exchanges are illegal gambling and that participants could be subject to penalties for mail fraud. Pyramid schemes are illegal, either by mail or on social media, if money or other items of value are requested with assurance of a sizeable return for those who participate.
These exchanges come in all forms; wine, books, kid's toys... so just stay away from all of them.