Intervals.icu analyzes its users rides, runs, swims and other athletic activities, and then provides athletic performance analytics. It also has a social media aspect, in which you can follow your friends, chat and help them to progress.
It mainly uses Cassandra as a key-value store. It stores the power, heart rate and other stats for each ride, run etc. It is data that is usually written once only, and queried by primary key. There is a lot of it, but mostly only the new data is queried. Takes lots of disk space, but not much CPU or RAM.