Probably because server usage alone will be $50k/month for just few million users. Customer oriented products (B2C, C2C) rarely makes money in initial days.
Just because Facebook began that way doesn't mean its replacement must.
Google Docs was free when Word was downloaded and paid for.
Whatsapp was paid for when other chat was free or included in your contract.
There are many business models and ways to grow. It's hard, but I think you could bootstrap a Facebook replacement... over a very long time!