I answered those talking points here:

https://news.ycombinator.com/item?id=33020993

I disagree that stateful based sessions are simpler.

I disagree size of cache is a red herring.

You don't have to be FB to want to optimize your memory usage.

In the end I prefer the more efficient (mostly stateless) solution and it's really easy to implement.

But to each their own of course.