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).
As BTS and now Steem have shown they are extremely talented, and could easily create something workable from scratch, but yes I agree they need a credible business plan to create a serious competitor/player going forward.
More importantly they need a sustainable business model to fund it all and they don't have one imo.
They are essentially hoping they can fund their expenses by diluting Steem holders. The only way this could work absent a genuine revenue source is if it became widely adopted as a limited currency like BTC which is very unlikely given their initial distribution and it's huge inflation rate. (Alternatively they are perhaps hoping they can make their SBD (Steem Dollar) very popular, which might create new independent demand, but again given the centralized nature of the Steem backing for it, it's unlikely to become trusted/popular either imo.)