For steemit to overtake reddit it does not take much I think, given that the use case is very similar and that the rewards are only there for Steemit. I think that Steemit also has the chance to attack Facebook, but this will not be so easy. The functionality is clearly not the same (yet) and you also do not have the same privacy possibilities you have at Facebook (I could not image me favoring Facebook for its privacy options). Due to the blockchain it will also be difficult to make these privacy options possible on steemit I guess. On the other hand, most Facebook users do not care anyway about privacy ;-)