MongoDB users vs Cassandra users
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We have compared MongoDB to CouchDB a while ago. The MongoDB has several advantages, speed, robustness, easy query, and deployment flexibility.
Cassandra, from FACEBOOK, is another NonSQL database, which has several unique advantages, linear scalability, massive write performance, and low operational costs, source.
Without digging into much technical details, the notable users of these two databases are shown below, so you get some sense of real applications of these two databases.