Steemit is a new site, relatively with a smaller network. Also the interface is quite poor for it to compete with facebook. however, its more like a blogsite and decent at that. the incentive to get people on to the network was money which did not really go down well and also, money is cryptocurrency and most people haven't yet adopted that. they need to learn about crypto, how to move steem to an exchange and how to cash out. With increasing crypto adoption and understanding of exchanges, people will come to steemit lock stock and barrel.
moreover, this is great time to create decntralised versions of fb, quora, reddit etc, right now, so that when people are more informed, we have the sites ready and we see exponential user growth.
right now with little crypto understanding, adoption will remain low as the incentive for people to move is not getting realised. however, steemit has done well to attract early adopters of tech i believe