Yes it does (look through 50TB of data), and how would you design it? It has to access all of your friends and find their postings. Robert Johnson gave an excellent talk on facebook's design two weeks ago at OOPSLA (it should be in the ACM digital library soon). He stated that there is no clear segregation of data, the (friend) network is too connected and extracting groups of friends isn't possible. Basically they have a huge mysql farm with memcached on top. Loading an inbox will hit multiple servers (maybe even a different server for each of your friends) across the farm.