Steemit won't replace Facebook. It doesn't do the same things, it's much closer to a blogging platform like Medium.
Something else might, for instance dMania seems like it could replace Imgur and it's built on Steemit... but Facebook is fundamentally different from either.
Facebook is how people maintain connections with real-life friends and family, while Steemit/Reddit/Imgur/dMania and others are all internet communities.
Things Facebook provides that would be difficult, if not impossible, without changing Steemit:
Friend lists (as opposed to followers)
Groups
Privacy settings (everything on Steemit is public)
Local events (calendars, planning, get-togethers with friends)
You could argue that these things could be implemented, but it would fundamentally change the nature of the site. Plus if it were "friends" rather than an internet community everyone would just upvote their friends' constantly for the rewards.
Privacy would also be really tough since everything is in the blockchain. I mean yeah, Zuckerberg might be spying on everything I post but I don't have to worry about my employer browsing all my photos from last weekend. (Who am I kidding, I'm old... I have no pics from last weekend. :( )
Plus companies like Google have tried to take down Facebook, and their attempt was much better than FB... and they failed. I don't see it going away anytime soon. Even the millenials with their snapchats and their instagrams (read that in Mallory Archer's voice) still use Facebook I think.
Moreover, stop trying to compare Steemit to other sites. Especially Facebook.
Steemit is awesome in its own right, and it's its own thing. Let it be awesome and stand alone.
You are totally right, MySpace was very different from Facebook, but it lost. I can't compare Steemit to Facebook because it really is a redicilous comparisson in terms of functions. But I do remember when Facebook became widly adopted thanks to their 'share' politics. There was a shift in the way people were thinking about how the web connects us. Now we have new platforms who reward the content creators and if a miricle happens maybe the same shift will happen. Maybe the real question is will we still be using platforms that sell our data instead of reward us.
Thinking it through what might take down FB is a portal that links in your other social services, if all said services are willing to create APIs to work with each other.
So, for example, you'd post your pics to Instagram/Snapchat, your ramblings on Twitter and your longer blog posts on Steemit.
The FB replacement then would be where people manage their friend lists and other things I mentioned, while the feed itself is populated from Instagram/Snapchat/Twitter/Steemit.