if you go for google scale, Hbase, Apache Spark, Redshift are more along the line what you gonna need. Even with services like docs.treasuredata.com/articles/quickstart it's still just for long term storage.
You need a high write throughput and if you think of analytic system I would go for a document based storage system since every request can be different and you probably need to scale on geo locations. So no pgsql it's possible yes ... but it's not it's purpose you can use it for the administration parts and things that require consistency.
Cassandra is a column based system it's nice and probably good for this usecase but I still would put a processing pipeline in front of it. something like

but that's just a basic idea, you need to think of fallbacks based on CAP / PACELC and you need a queue for buffering so you have realtime updates that can be delivered to the end user as well as sending it to the API.
We're talking eventbased / stream based design that has to be designed async with eventual consistency.
those are my initial thoughts.