The president at the start-up I'm working for, Lorenz, has been ranting and raving about how awesome Facebook is...so much that we're starting to wonder if he is secretly working for them. Though I still don't quite understand what the buzz is all about yet, he recently found and used their polling feature that I find amazing.
For as little as $6, you can poll Facebook's 30 million users to get insight on questions you may need answers to without going through all the trouble of usability testing or traditional (and expensive) polling methods.
We tried a few questions and got several hundred responses very quickly. Overall I find it extremely useful and will definitely be using it a lot to assist in some of those difficult design decisions.
UPDATE:
Ok now that I've been using Facebook for the past couple of weeks, I can see the light now. From the time I registered to now, I've added about 5 friends on my own, but other friends and friends of friends have quickly found me and added me to their friends list. I was quite amazed at how quickly and easily they help you and your friends connect (via your IM, email contacts, etc...). Once you have a decent amount of friends you're connected to, Facebook gets really awesome.
Facebook is one million times better than myspace. Granted I'm not a popularity-starved teenager who wants to pimp out my profile beyond the point of legibility, but the usability and thought they've put into every detail in order to enable a great social network is really impressive.