Sometimes people used these informal ways of _re-homing because they didn't want their social worker to know. Or they didn't want the child to stay in their home community and be a constant reminder of what wasn't.
We applied to adopt a child who had been originally from another country and the local family who brought them here had decided the child wasn't working out. Our adoption worker said we couldn't adopt her because the family didn't want her nearby and we lived close.
The family thought it would be hard on their other kids to see their what once was sister on the street knowing she lived with another family. That is true but I always felt it was unfair that she was the one who had to move away from her school and community. That there should have been a better way.