Ryan Albrecht digs into how efficient browser caching is on Facebook.com. They release code twice a day, breaking cache as they do, so they were curious if that was too often for browser cache to be efficient.
After collecting data, they are seeing 44.6% users getting an empty cache, which they determine effective:
The best practices tell us to use external styles and scripts, include Cache-Control and ETag headers, compress data on the wire, use URLs to expire cached resources, and separate frequently updated resources from long-lived ones. All of these techniques work together on any website, not just one at Facebook scale.
I’m not sure at what level they would be determined not useful though. Seems browser caching is an easy enough thing to implement that if 90% of users got an empty cache it would still be worth it, if just for the speed of that session alone.