It's remarkable the effect even short TTL caching can have given enough traffic. I recall once caching a value that was being accessed on every page load with a TTL of 1s resulting in a >99% reduction in query volume, and that's nowhere near Facebook/internet backbone scale.
yep, prepriming the cache rather than passively allowing it be rebuilt by request/queries can also result in some nice improvements and depending on replication delay across database servers avoid some unexpected query results reaching the end user.
In the past I was the architect of a top 2000 alexa ranked social networking site, data synchronization delays were insane under certain load patterns high single low double digit second write propagation delays.
