Twitter search serves over one billion queries per day. The unfortunate tsunami in Japan caused such search query load as users experienced significant delays. About the same time Twitter released their latest search technology (Blender) based on Java/Lucene. As a result Twitter were able to see a 300% drop in most search times and significantly the CPU load on front-end servers was cut by 50%.

More details can be found on Twitter own blog http://engineering.twitter.com/2011/04/twitter-search-is-now-3x-faster_1656.html