[staunch]

They're running a massive web app. That means they have all the usual stuff any site would need. I imagine it's mostly just a matter of adding enough servers to handle their growing load. They must do quite a bit of data crunching as well.

Their third party app infrastructure must be quite significant. They're proxying and processing huge amounts of traffic. Almost certainly doing it with a custom server app that's probably quite interesting technically.

A huge portion of that $150M must be going towards network infrastructure (where it's damned easy to spend millions) and datacenter space/power, etc.