It's interesting work, but it's not really 'big data'.

"Every day, we collect around 8 million data points on exercise and video interactions, and a few million more around community discussion, computer science programs, and registrations. Not to mention the raw web request logs, and some client-side events we send to MixPanel."

OK - 8 million records per day. Let's double that for the argument's sake.

Even if they were fairly fat records (1Kb), that's only 16Gb / day. That makes it around 2 months / TB.

I can easily put together a machine with 20TB of storage and run a traditional free relational DB (or even a single free node of Greenplum) and store more than 3 years of this data.

Then bang against it with SQL. Transactions are free.