The big advantage of software though is that you don't have to maintain a fixed ratio of developer:user like that, though. Sure, your employee count should increase as user count does, but far less than linearly.
I'm not sure this is in practice true. Twitter have to start writing or tuning their own scale-specific software, need more analytics, application security, developer productivity, partner and sales engineering etc. proportionally than they would do at 1% of their size. Certainly they could run 100 Plurks with many fewer than 1000 engineers but I'm not sure that's the best way to characterize Twitter.
