* http://www.quora.com/What-caused-Foursquares-downtime-on-Oct...
* http://blog.foursquare.com/2010/10/05/so-that-was-a-bummer/
* https://groups.google.com/group/mongodb-user/browse_thread/t...
Someone on this thread suggested that Foursquare's performance problem is related to the calculation of badges. I'm not sure where this idea came from. I haven't seen any mention of the issue being related to badges in first or secondhand sources.
That said, Mongo DB's map/reduce operation would not be a reasonable solution at this time. Mongo DB's map/reduce performance is, at present, somewhat lacking because it runs via the javascript engine which is currently single threaded. I know there are plans to improve performance of Mongo DB's javascript engine by switching to V8, but I don't know if V8 is multithreaded.
Often design decisions that look bad in hindsight get baked in early. By the time you realize that an alternate design would yield better performance, there may be too much data to migrate so you just have to live with it.