I'm specifically addressing the original parents straw man.
> There are maybe, what, a thousand people in the U.S. that have worked on network administration for something GitHub's size?
I didn't intend to deflate any nerd egos, just putting things in realistic terms. GitHub is cool. It's the biggest in its industry. It's definitely an interesting problem. But it's still hosted within Rackspace, which in itself isn't that big.
What stops someone from having multiple zones within a datacenter interconnected with suitably large LAGs thereby avoiding being limited by the port capacity of any one device?
Why is the exact number of servers important to the topic, or is it just to satisfy your curiosity? :)
[1] http://www.aristanetworks.com/en/products/7500series