It is hard to say that the Steemit can destroy/disrupt FB. But there is decline in the use of FB. People are migrating to other social networking sites.
Steemit have attracted many people in very less time may be due to its monetary system but it is great because I belief People always invest their energy from where they can get some return. And I hope the way the Steemit is moving definately it will be in the list of top social networking sites.