Caveat: this isn’t about failover, security, or cloud computing.
Imagine you have 65 million registered users and you need to provide custom content for each (beyond “Hello, Bob”), say real time subscription content from tens of thousands of sources containing millions of posts.
Don’t try to show everything at once
You probably won’t be able to show everything to the user at once. Depending on your load and back-end systems, you may be lucky to simply let the registered user know that they have something to see.
Most of your users won’t be logged in
Most page views (at least to a homepage) won’t be on logged-in users: many will be first time users, or users who haven’t visited very often. You’ll need to hit your back-end servers with a specific ratio of total visits, something you can measure early, and use as a baseline for how many servers you need to scale.