Lucumo: It's not the public domain. Forums usually have TOS like this one:
"By registering an account at X and participating (including but not limited to submitting posts or private messages on X), you hereby grant X and our licensees, distributors, agents, representatives and other authorized users, a perpetual, non-exclusive, irrevocable, fully-paid, royalty-free, sub-licensable and transferable (in whole or part) worldwide license under all copyrights, trademarks, patents, trade secrets, privacy and publicity rights and other intellectual property rights you own or control to use, reproduce, transmit, display, exhibit, distribute, index, comment on, modify, create derivative works based upon, perform and otherwise exploit the messages posted or private messages sent on X, in whole or in part, in all media formats and channels now known or hereafter devised, for any and all purposes including entertainment, news, advertising, promotional, marketing, publicity, trade or commercial purposes, all without further notice to you, with or without attribution, and without the requirement of any permission from or payment to you or to any other person or entity."
Of course, but "non-exclusive", I would think, means that data is no longer private.