The developer of Steem is one of the most talented in crypto so if the Steem vs. Reddit model worked they could certainly rapidly develop a Facebook type extension on the same principles where the most popular content is rewarded. (You could actually create any model that paid new users for sign ups/content/other and in the process attract lots of new users and look like you have a popular and growing site.)
But Facebook has 1000s of employees. You don't develop the breadth and depth of Facebook's ecosystem and thus feature set overnight with one developer. They need a plan to build a company with many developers in order to reproduce Facebook. Or they need open source it and hope the ecosystem grows with network effects. But their license on Steem doesn't even allow forking. And Steemit isn't even open sourced (yet).