When a photograph is in the public domain, it means that there is no copyright, and anyone may use, copy or distribute the photo for any purpose. There is nothing about posting photos online that inherently places them in the public domain.
You own the copyright to photos you take from the moment you take them. When you post them online through a third party website or social media platform, you agree to the terms of service of that website. Many major image-sharing websites have terms of service stating that you continue to own the copyright to photos you post, but you grant the website a license to copy or distribute them. This may be sublicensable, so that the service can give others permission to reblog, retweet, or otherwise share the photos. Other websites, such as Unsplash and Pixabay, are set up specifically for photographers who want to release their photos into the public domain. Therefore, by posting on these sites, you do agree to terms of service that place your photos in the public domain.
Public Content When you post content on your Facebook page, you have the ability to configure privacy options regarding who can view that content. While some content may be strictly controlled, the majority of content is simply posted as "public." According to Facebook's Statement of Rights and Responsibilities, any image posted with the Public setting carries the implicit acknowledgment that said photo is available for access and use by anyone -- including entities not on Facebook. While permission for use is considered polite, it is not a prerequisite. Privacy-Controlled Content Facebook posting also includes the ability to control who can see content. Privacy-controlled content is something of a grey area, as the content is still being posted to Facebook, but only to specific users and therefore not "publicly" per se. If the photos are considered an invasion into the privacy of the subject, they may file a request to have the content deleted from Facebook. However, it can be assumed that anyone allowed access may make use of it with limited permission.