That could be...Steemit is the groundbreaker but might not end up being the most potent.
I feel Zappl and D.Tube are going to add more users to this blockchain than Steemit will. Those two are going to have millions of users apiece. Of course, they are all tied together but it is easier to use both of them when new as opposed to Steemit.
Of course, with so much development on here, the second one could well be right where we are sitting.