What FR really needs is a clustered database, that automatically replicates data across servers. Alternatively, use of a distributed cache like memcache would probably make a big difference. It's how large sites like Facebook, Wikipedia, YouTube, Reddit, and other similar websites handle large amounts of dynamic data.
There's no simple solution. Faster servers can help, but you run into other limiting factors if the application can't handle high levels of concurrency.
Indeed, all user authentication data and the most recent week of posts would probably fit easily in a few gigabytes of RAM. The challenge is how to architect access to it.
You are right that DB clusters and memcache would work. Even simple filesystem caching would probably work. A reverse proxy cache like Varnish would probably take some of the heat off.
Ah, you make good points.
How well does memcache work with Perl? At least that’s what script lang this site is largely written in AFAIK.
I’d also wonder if Perl is more server-intensive than something like PHP.