It may not be hip, but it handles tens of thousands of users, and tens of millions of (non-cacheable) hits daily, with just a few commodity servers --

http://improvely.com : Nginx + PHP-FPM (5.4) + Percona (MySQL 5.5) + Redis

http://w3counter.com : Nginx + PHP-FPM + Percona + Redis + Node.js (for live dashboards with lots of connections)

Hosting by SoftLayer. I surveyed the hosting choices of over 300 YC-backed companies; Amazon EC2 and PaaS running on EC2 was the most-used, with Softlayer in second place. If you run your own servers, they're the best hardware, public/private network infrastructure and support you can buy, IMO.