It is essentially impossible to forfeit copyright in the US. The "including copyright symbol" requirement is gone since March 1, 1989. Works published in the US prior to that date suffer a different fate that anything subsequent (one can correct an error within a certain time frame, long since past). Publications before January 1, 1978 had an absolute requirement of a copyright notice.
The benefit of a copyright notice, currently, is that if the notice (symbol or words) is missing, the infringer may be able to argue "innocent infringement", i.e. not knowing that the work is protected by copyright, which could reduce the size of their statutory damages liability when infringement is found (17 USC 504(c)(2). With a notice, per 17 USC 401, such an argument is not available.
US copyright law does not have a statutory mechanism for renouncing copyright. The typical way to do so is to grant a perpetual public license basically saying "I renounce all copyright to the work". However, that would not be a very good way to put it, because it still doesn't give others permission to copy. Hence some license like CC-0 puts it out there for anyone to use as they see fit.
This probably works, though there is a minor complication that "public domain" licenses are often bare licenses (it's not contingent on others doing something for the author), which could be revoked. Then one would have to rely on estoppel to prevent the re-assertion of copyright.
As for Facebook (or SE, or anywhere else that I've ever encountered), everything is licensed in some manner. Anything you put on Facebook grants Facebook "a non-exclusive, transferable, sub-licensable, royalty-free, worldwide license to use any IP content that you post". I don't know the details of Facebook, but that means they can grant any user a license that has all of those restrictions. The license is not perpetual because "This IP License ends when you delete your IP content or your account unless your content has been shared with others, and they have not deleted it", and that latter part says that it is effectively perpetual, if you make it available to anyone.