HTTP proxy type load balancers are slugs in comparison
Scaling app servers to nearly unlimited size is easy to explain but really hard in practice. It basically amounts to this:
1) Balance requests using DNS anycast so you can spread load before it hits your servers
2) Setup "Head End" machines with as large pipes as possible (40Gbps?) and load balance at the lowest layer you can. Balance at IP level using IPVS and direct server return. A single reasonable machine can handle a 40Gbps pipe. I guess you could setup a bunch of these but I doubt many people are over 40Gpbs. Oh, and don't use cloud services for these. The virtualization overhead is high on the network plane and even with SR-IOV you don't get access to all hardware NIC queues. Also, I don't know of any cloud provider thats compatible with direct server return since they typically virtualize your "private cloud" at layer 3, whereas IPVS actually touches layer 2 a little. Do yourself a favor and get yourself a few colo's for your load balancers.
3) Setup a ton of HTTP-proxy type load balancers. This includes Nginx, Varnish, Haproxy etc... One of these machines can probably handle 1-5 Gbps of traffic so expect 20 or so behind each layer 3 balancer. These NEED to be hardened substantially because most attacks will be layer 4 and up once an adversary realizes they can't just flood you out(due to powerful IPVS balancers above). SYN cookies are extremely important here since you're dealing with TCP... just try to set everything up to avoid storing TCP state at all costs. This also means no NAT. You might want to keep these in the colo with your L3 load balancers.
4) Now for your app servers. Depending on if you're using a dog slow language or not, you'll want between 3 and 300 app servers behind each HTTP proxy. You don't really need to harden these as much since the traffic is lower and any traffic that reaches here is clean HTTP. Go ahead and throw these on the cloud if want