How many servers and what does each one do? I am just wondering for future reference Just wanting to know, because I am interested in networking and wanting to know how large websites run their websites in the best way.
Every large site has different needs depending on it's income level, activity, and type of content. High activity with lots of dynamic content is going to be more load than static content.
Forums happen to be one of the most intense scripts for a site you can run. A single page can have dozens of queries.
HF is run on multiple servers and I utilize a CDN to serve static content. It has a dedicated DB server with 24GB ram and dual quad-core processor. The web server is getting old but it's an 8GB quad-core. It's running fine but will be replaced in the next few months. That's the basics.
I've personally spoken with many large forum owners and some spend 20k-100k per year to host their forums. I'm still under 10k per year but that's only because I'm the sysadmin and spend a great deal of time on it.
DP for example runs a very serious rack system in a datacenter. But they run more than just a forum. He also earns a great deal more than I do so he can justify the costs.
I don't see a great need to get a rack in the next 18 months but after that it's possible I may benefit from a half-rack with my own colocated cluster.