Actually, from Rails, to ASP.NET MVC, to ASP.NET MVC again ("because it was getting too complicated"), then to Rails.

I'd also be very hesitant to use MongoDB as a main datastore as it has no durability model - the 'D' in ACID. I hope they've at least changed the setting, or don't mind losing their last 60 seconds of writes when their EC2 instance goes down.

http://www.mongodb.org/display/DOCS/Durability+and+Repair#Du...

Note that Mongo compares itself to MyISAM, but that is infrequently used now because of its lack of durability amongst other things - no transaction support, etc (most apps use InnoDB).