Ask HN: Best scalable way of storing sessions?

Like many, our startup began by storing sessions in MySQL (InnoDB). This has become inefficient -- basically requiring us to write to the database on every page load.

Many people suggest using Memcached to store sessions but one of the core memcached developers recommends against that here: http://bit.ly/dZCT

Storing sessions in a cache is not really what they were designed for: caches can evict data, and if you only have one memcached instance, your sessions would be contending for space with the rest of your cached variables.

My question is: what do you use to store sessions on your high-traffic application where speed matters? What is the very best solution to this problem? What do places like Facebook do here?