Many people mistakenly assume that MapReduce is only useful when dealing with Google or Facebook scale datasets. Nowadays, MapReduce is simply a convenient data access pattern, particularly when your data is spread across multiple machines. For example, Riak employs the MapReduce paradigm as its "primary querying and data-processing system." [0]
Clearly, Riak does not expect each and every query to run for hours and involve TBs of data. MapReduce is just a straightforward way to process distributed data.