Ilya Grigorik – Open Source Search with Lucene & Solr:
"Salesforce, LinkedIn and just recently Twitter are all examples of large-scale Lucene deployments that many of us use on a day-to-day basis. Salesforce started with Lucene back in 2002 and today manages an 8TB+ index (~20 billion documents).
[…] iTunes is another notable user, said to be handling up to 800 queries/sec, and at PostRank we are currently managing a 1TB+ index (growing at ~40GB a week) with over 1.2 billion documents."
(Via Patrick Durusau.)