Why continue to develop what you already have with developers you already employ? Why not spend billions of dollars acquiring software and engineers you don't need? You have to think like an executive. If they aren't "buying" stuff they aren't important. There is a reason that Facebook has 13,000 employees now. It has nothing to do with the business.
I doubt they cared very much about the software or the developers in the valuation. In that respect I think we agree; they didn't really need those things. What they needed and what they bought was 1 billion active users.