It is hard to give that detail without knowing detail about how many concurrent users are using apps. If we have more breakdown then it is easy to give that detail.

Without much requirement detail, On the safe side - My guess is three Linux server cluster (Specs 2-CPU and @4GB RAM ( @2500$) )should be enough to server content for high usage site like facebook.

It is purely guess based on my limited knowledge of facebook. If you have any specific detail like

- Avg Concurrent users - Peak time concurrent users - Apps software platform ( java app server, php, .NET ?? )

then that will help.