Facebook
We use Hadoop to store copies of internal log and dimension data sources and use it as a source for reporting/analytics and machine learning.
Currently we have 2 major clusters:
A 1100-machine cluster with 8800 cores and about 12 PB raw storage.
A 300-machine cluster with 2400 cores and about 3 PB raw storage.
Each (commodity) node has 8 cores and 12 TB of storage.
We are heavy users of both streaming as well as the Java APIs. We have built a higher level data warehousing framework using these features called Hive (see the http://hadoop.apache.org/hive/). We have also developed a FUSE implementation over HDFS.