OpenAI CEO Sam Altman Justin Sullivan/Getty ImagesAs Reddit prepares to go public, a new filing with the SEC has revealed its biggest backers.OpenAI's Sam Altman is the third-largest shareholder, the filing revealed.It was previously known he was a Reddit investor — but not to this extent.We learned a new detail about Sam Altman Thursday: just how much he supports Reddit.In preparation for its highly-anticipated IPO, the online social forum filed an S-1 form with the Securities and Exchange Commission on Thursday, detailing the company's financial background.