I think we are still a little while away from seeing that kind of take over. Steemit needs a little upgrade and perhaps an app/mobile version along with the ability to get notifications. Steemit Definitely could become as popular as facebook or instagram, but its function is totally different. Facebook and Instagram are based around posting pictures and commenting on other’s along with sending messages back and forth. Steemit is more of a bog site.
Unless we were to completely reevaluate the concept and purpose of steemit, it will not replace facebook or instagram, though it could rival it in popularity