Inside Google, MapReduce is used for 80% of all the data processing needs. That includes indexing web content, running the clustering engine for Google News, generating reports for popular queries (Google Trends), processing satellite imagery , language model processing for statistical machine translation and even mundane tasks like data backup and restore.
The other 20% is handled by a lesser known infrastructure called “Pregel” which is optimized to min erelationships from “graphs”.
While you can calculate something like ‘pagerank’ with MapReduce very quickly, you need more complex algorithms to mine some other kinds of relationships between pages or other sets of data.
See the following video tutorial to see an Introduction to Pregel