It's spreading pretty rapidly in my circles. It's used by a lot of popular sites, such as Github, Soundcloud, Stumbleupon, Quora, Foursquare, Etsy, etc.

They have some case studies:

http://www.elasticsearch.org/case-study/

eg. http://www.elasticsearch.org/case-study/github/ http://www.elasticsearch.org/case-study/soundcloud/ http://www.elasticsearch.org/case-study/stumbleupon/

For what it's worth a gg-tends comparison between solr (the main ES competitor I know of) and ES itself.

http://www.google.com/trends/explore#q=elasticsearch%2C%20so...

TLDR; they are roughly equal in popularity according to gg-tends, ES growing very fast.

My impression is that Elasticsearch's growth is coming from being more popular outside of the Java ecosystem.

I run the Elasticsearch Berlin UG and we have a steady and fast influx of new people. I've rarely seen communities grow that fast. The community is great and active and Elasticsearch is caring a lot for that.

http://www.meetup.com/Elasticsearch-UG-Berlin/