De facto attempt at registration. You post a picture of a gun on Facebook, they have your name, address, telephone numbers, they know you have a gun, the type and have an idea of how many you have. The NRA and other Firearms groups have been battling against this for decades. The Serial number is somewhat irrelevant. They know your friends and who you associate with… This is one of many reasons I refuse to set up a Facebook account.
This data can now be used to dox gunowners, given to the government, used to ban you from the platform. Nothing good will come of this.