Eugene Letuchy (a Facebook Engineer) talks about Facebook Chat

"One of the things I like most about working at Facebook is the ability to launch products that are (almost) immediately used by millions of people. Unlike a three-guys-in-a-garage startup, we don't have the luxury of scaling out infrastructure to keep pace with user growth; when your feature's userbase will go from 0 to 70 million practically overnight, scalability has to be baked in from the start. The project I'm currently working on, Facebook Chat, offered a nice set of software engineering challenges:"

• Real-time presence notification
• Real-time messaging
• Distribution, Isolation, and Failover
• Glueing with Thrift

Here is the original post by Eugine
http://www.facebook.com/note.php?note_id=14218138919