Our application servers run Django with PostgreSQL as our back-end database. Our first question after deciding to shard out our data was whether PostgreSQL should remain our primary data-store, or whether we should switch to something else. We evaluated a few different NoSQL solutions, but ultimately decided that the solution that best suited our needs would be to shard our data across a set of PostgreSQL servers.
IDs should ideally be 64 bits (for smaller indexes, and better storage in systems like Redis)