Are you sure about that hint?
The AWS case study linked below
http://aws.amazon.com/solutions/case-studies/foursquare/
mentions "stream[ing] hundreds of millions of daily application logs each day." These logs must be at least 0.5K w/ metadata (probably more), so we're talking about...what, 100GB/day of application logs? Alone?? You're not persisting those using cron + MySQL--they likely use Kafka/Kinesis, many threads/instances subscribing to topics, and a large, horizontally-scalable db or Redshift to manage them.
Note that I haven't even addressed querying those logs (I believe 4sq uses Hive, the case study mentions Tableau). That all costs money.
We also haven't mentioned reviews for their millions of locations, user auth, check-ins, app development (web/iOS/Android). Each of those is a problem for a company with a MM-level user base.
Do you have some kind of intimate knowledge of their monthly engineering costs?
EDIT: 4sq website says 2M legit businesses, 65M total places.