The main reason of not replacing facebook by blockchain system at this time, is the lack of knowledge in numbers of people. many people still have not awarenwss about the steemit and other blockcahin platform, so that, they are unable to become a part and trust on such kind of platforms. people only want to get entertained on internet instead of adding some technical values in there life. but, after 2, 3 years, i 100% sure that steemit blockcahain system will take the place of social medias life FB, Tweeter and many other. and now the main uestion is that how, when and who Dominating the youtube in future?