Comment by opendomain

There are other reasons to choose NoSQL For example, when Craigslist was using mySql and they had to change their schema, it took MONTHS to facilitate the change across all their slaves. You can also have a mixed strategy of using both RDBMS and NoSQL to achieve consistency while being able to be flexible to architecture changes. Lastly- have you looked at total overal cost? Setting up a large cluster with mySql will have a large operational cost and it may not be partition tolerant so if the wrong servers go down, it may cascade to your whole data store.