Clive Thompson has written a great article for the New York Times about what makes Facebook and Twitter special. I've tried explaining both to people, and it's very hard to convey what makes Facebook different from earlier social networks, or what makes Twitter different from having a blog. This article explains it quite well.

I've worked on two virtual worlds in the last year, and in both I added features similar to Facebook's News Feed. It's just such a great idea, especially when tied in to buddy lists and easy invite systems. But I think you need to have experienced how it changes how you relate to people to appreciate it.

Even if you keep up to date on this stuff, I recommend reading the article. It goes further than just saying that Facebook has a news feed or that Twitter is cool.