Following reports that it may either buy or invest in photo blogging site Tumblr, Yahoo (YHOO) on Fri. announced that CEO Marissa Mayer will unveil a mystery product at 5 p.m. ET on Mon. Tumblr reportedly has a value of at least $800 mil, and an AdWeek report estimated it could fetch as much as $1 bil. Yahoo has bought several companies since Mayer became CEO last July, including Summly, Astrid and Jybe. Shares fell 0.2% to 26.52.