NoSQL Is Not An Option
Today, NoSQL databases are actually distributed data stores which are designed with very large scale data access requirements--such as Facebook. Cassandra, the most popular NoSQL database, got its start as Facebook's proprietary database. The social networking company made it open source in 2008, and opened the doors for others to begin using it.
Large sites, such as Twitter, handles huge amounts of data storage and data writes each day--in Twitter's case, 7 TB (that's terabytes) of data per day. Handling such a large amount of data can be difficult for relational databases. The difference is that NoSQL's distributed data stores are able to scale the data by distributing it across multiple nodes horizontally.