I can't claim to know the specifics of your application, but consider that a single upvote invalidates however many thousand copies of a given page. An action i

I can't claim to know the specifics of your application, but consider that a single upvote invalidates however many thousand copies of a given page. An action in a Facebook game might invalidate a page for 10 or 20 or 50 users. An upvote, (or a comment, or an edit, or an upvote on a comment) invalidates that page for upwards of 300k+ users. Combine that with the rate at which reddit's pages change, and you're talking an obscene amount of CPU time spent building fragments/pages to stick into your cache. Scope of visibility of a change means a lot. You obviously know how to scale an app if you were pulling 16 million pageviews a day, and I don't intend to discount that at all. I just mean to point out that while fundamentally the same problem, reddit has to deal with a version of that particular problem that most applications don't begin to approach.