How did Justin beiber became famous just by youtube?

People say that JB became famous by singing on youtube but how many people really watch any other person singing without any stars at the begining and if sme. People watched it also at the starting thr. Shud. Be just at most 50-100 views and the people who watched it also,all can't be in hollywood so just youtube can't make him famous if it does thr. Are millions out thr. Like him uploading videos on youtube,so what is the real thng. Behind this???