Relational databases are not the right choice for large-scale data stores serving large number of users because:

- they provide unnecessary features at high cost (e.g. database-wide transactions),

- they don't provide features that are essential (e.g. scalability, distribution, graceful degradation).

By contrast NoSQL data stores like Bigtable are highly scalable and can easily be stacked over to provide more features if needed (see e.g. Megastore, http://pdos.csail.mit.edu/6.824-2011/papers/jbaker-megastore...)