It is not exactly a fair to compare Steemit and Facebook. Steemit is a social blogging platform while Facebook is a social sharing platform. I will think Medium is a fairer comparison with Steemit.
In any case, I am certainly spending more time on Steem than on Facebook. For a couple of good reasons. First, I get paid when I write on Steemit. Second, I think Steemit has more quality content than what I can see on Facebook.