Facebook will never go away, though. There are plenty of people on it who use it simply to stay in touch with others. Additionally, some people crave the centralized setup because it lets them block content they don't agree with or find "offensive." Facebook lets them do that by reporting things and getting things removed. They won't want to let go of that control.
Admittedly, a big draw of Steemit is income. However, I also love the concept of people being able to post what they want with less regard of public denouncement by trolls and haters. There are far too many of those on Facebook, many of which are in that group that want to maintain control for themselves.
So while I do believe Steemit will continue to grow, there will always be something else for those who can't handle free thinking and tolerance.
Just my 2 cents.