Ask HN: I built a site that just went ridiculously viral. What do I do now?

Sites like this typically see eCPM between $0.10 and $0.30 for advertising. So you're looking at around $30/day revenue right now.

Consider switching to EC2. This will allow you to easily scale up to a more powerful server if growth continues or down to a less powerful one if traffic tapers off.

Depending on how you set things up, a common bottleneck for Facebook applications is API calls. Ensure you are not making any API calls from Rails, since this will cause Rails processes to hang until Facebook returns results. With high traffic this could easily cause your server to become overwhelmed. Move all API calls to Delayed Job or another background processing system if you haven't already.

It looks like you have relatively few remotely loaded assets, which is great. You can likely speed things up more by making sure that your database is well optimized for the server (Make sure query cache is enabled, and you have indexes on the right columns for example), and identify any actions that take a long time to return.

As for what to expect in the future, growth will not be infinite. There are a finite number of people on earth that are going to love your site and Facebook integration is a fantastic way to reach nearly all of those people in a very short amount of time. After a while most of those people will get bored and you will see traffic begin to fall. The best ways to address this are by consistently releasing new features to reengage users or by referring your users to other similar sites that you build.

I suggest migrating it to Heroku. Push your code, copy over the database, and crank your dynos up. They charge by the hour, so as an example if you have ~10 dynos running (roughly equivalent to 10 thin instances, but slightly better) for 5 days (or however long it takes for the initial traffic surge to subside), you're only going to pay about $50. Then, you can turn down your dyno count and pay a reasonable monthly fee.