Facebook

Bill that's really a good q as I've had similar experiences. In one case a forum software got mixed up between my handle and my username

…..making it impossible for me to register a grievance about it. However with other such sites it might be possible nevertheless to contact their Admin

Interesting to note that Facebook is a bit messed up also. I had attempted to join but my membership was suspended when somebody in Europe or the Far East tried to pirate my username; so they won't let me back in unless I establish a different password

But after having had to keep track of 50 different passwords I had quit all such sites and forums that wouldn't accept my usu for one reason for another

Good luck