When we look at different sites, Steemit looks the most like Reddit. Since the interface is much nicer, and Steemit has a reward system that Reddit lacks, I predict Steemit will grow to the Reddit we know today and even further. (and so will steem & steem dollar rise in value)
I don't think Steemit will ever replace a facebook or instagram, because it can't compete with all the functions and well designed interface given on these sites. I do think it will be a nice addition to the current social networks because here is an active community eager to discover great posts!