Ask HN: Most scalable/cost effective method of implementing activity streams?

Instagram said they use a redis list and just trim it to keep the size down but what if you are following a ridiculous # of users? How do you know what is a good # to trim at and what if a user wants to go past your limit? Do you start them a second list to catch everything getting trimmed? Is there any way to implement this a better way with postgres/hstore? Does anyone know how twitter does it?