Great high level summary. Given current work scaling up a smaller site (3million pageviews/month) with more social networking functionality is that Stack Overflow ought to be pretty amenable to caching. Obviously it's pretty interactive so it's somewhere middle of the road. But what's a real cache killer are the types of per-user customization that Facebook does. Not to take anything away from Stack Overflow, but it seems like it ought to be served pretty well by standard techniques whereas something like Facebook obviously needs some juicy custom middleware. Would love to see an article about that.