How really big applications (such as facebook) scale their database
These companies use a mix of commercial products and new technologies developed in-house. Many are documented. Several are open source. The larger companies maintain separate research organisations to bring along the next generation of products. Many papers are available and make for interesting reading.

Broadly, the NoSQL movement was driven by these organisations' needs that could not be met by products available at the time. Cassandra, DynamoDB and LevelDB, amongst others, came from this movement. More recently NewSQL has emerged, giving global scale and ACID guarantees. One example is Google Spanner.