Look at it this way: People were writing letters to communicate until the telephone was invented. Then "slowly" people started to migrate from the telephone to internet services like skype, viber, messenger etc. Those services took away a part of the market share from phone service providers.
So every product/service seems to have a shell life. Facebook is no exception, although it's a way harder nut to crack. Still, there is a big possibility that Steemit will at least take market share from facebook (the social platform) or other social media platforms, if not completely disrupt them, like was the case for: letters/email, cart/car, candles(as light)/electricity etc.