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Mozilla's "Test Pilot" user testing system uses Riak. The Mozilla Metrics blog explains why they chose Riak, with comparisons to Cassandra and HBase:

Heroku actually published a really good blog post about this today.. it was on the frontpage earlier:

Riak's nice for redundancy and high availability stuff, and scales dynamically by adding instances.

May be a bit dated but right from the horses mouth: http://wiki.basho.com/display/RIAK/Riak+Comparisons