How is that even relevant? Images cost in bandwidth, not in computing power.
> Does Reddit find things only specific to your account
Uh yes, on every single submission and comments. The admins clearly stated that what used the most resources was the voting system. As well as every single user (whether you marked them as friends) and the list of links itself, which is extracted from the user's own list of reddits.
There is barely anything on a logged in user's page which isn't at least in part specific to that user's account.
> I could really go on for a long time
No, I don't think so, you've been 0/everything so far, it's time to stop.
Sounds like you need to do some STFU yourself, having clearly never come anywhere near this problem domain. Using standard tools to process and resize user-uploaded images can easily soak up all the CPU time and memory you could throw at it.
I recently spent a bunch of time rewriting a client's image processing pipeline fromt the usual O(n)-space ImageMagick crap that loads the full uncompressed image into memory a few times over to be O(1)-space doing streaming downsampling by exploiting the compression implementations of JPEG and PNG. It was more than worth it — even with the PNG implementation being a bit more CPU intensive now, it's a lot nicer having it operate in constant space without fear of swap and the OOM-killer.
Your personalized news feed and what you can see depends on the settings of all your friends and those friends.
Reddit is just a simple forum site...
Facebook also doesn't count all webserver HITS as pageviews, whereas most of Reddit's do count.
Beyond that, pageviews for Facebook may eat more resources (CPU/RAM) than Reddit on average due to photo uploading and other misc. things on Facebook. This means a server is working harder at 2pageviews/sec/server for Facebook.
Facebook have revenue, and profit. They can afford to buy more servers and have them sitting idle.
Reddit, should be trying to run on an optimal number of servers. Firstly, cheap hosting (Which is NOT amazon), and secondly less servers. 80 is just crazy.
As someone who's worked in a data centre before, I can say hardware problems only increase as you scale up. We had around 350 servers with an average of three drives each, and we probably went to the data centre to swap dead hard drives on average three times a week - SAS, SATA, or SCSI. Nevermind the time I spent testing motherboards, swapping CPUs out, etc.
Owning your own servers means dealing with maintenance and downtime. Virtualizing 10 servers on one physical server is great until that one physical server's RAM starts acting flaky, and then you have to take those 10 servers offline, or shift the VMs onto your other hardware.
Renting your servers typically means paying a monthly cost for the hardware, including RAM and disk, long after the fees have paid the DC back their costs.
For Reddit, being able to bring N extra nodes for X purpose (mysql, cassandra, web serving, etc.) with a few clicks in a few minutes is likely a huge draw for them. It means they can grow gradually (instead of having to shell out another $10-20k for a new server), and it means they can dynamically adjust to meet their needs. For example, if Monday is a busier day than Saturday by a significant stretch (and if their architecture allows it), they can bring more nodes online early Monday morning to handle the load. Take them offline until Thursday evening to handle the 'It's Friday, to hell with doing work' rush I'm sure they get, and so on.