Thats exactly what the facebook newsfeed is. A streaming feed of content, posted by publishers and users alike, and through likes, shares, and comments its popularity rises and it floats its way up the algorithm. Then when it gains reach, lots of people comment on it.

Facebook still has user to user messaging, Youtube does not.

Just because you have an algorithm that determines what should flow to the top of the main page, doesn't make it a social network.