Gaug.es uses Mongo. Its data model is pretty simple (just counters), but he has blogged about the approach in detail.
MongoDB Counters: http://railstips.org/blog/archives/2011/06/28/counters-every... http://railstips.org/blog/archives/2011/07/31/counters-every...
Kestrel: http://railstips.org/blog/archives/2012/03/05/misleading-tit...
Gets some decent traffic and works well.