Privacy: Another big selling point for Facebook is the privacy controls that they have put in place. By default, only people who have verified that they are in your same network (school, company, or region) can see your profile. That can be even further restricted to allow only friends to see your profile. MySpace also offers similar privacy controls, but Facebook's use of networks creates an increased perception of privacy. On MySpace, your profile is either public or exclusive to your friends, while on Facebook, it can still be exclusive to your friends, but it can also be open to people who it would actually be useful to network with. This has caused a lot of college kids to flock to Facebook, in search of a more closed network than MySpace.
News Feed: Facebook's News Feed feature, released in September of 2006, gives users quick and easy access to interesting information about their friends from the home page. Although this was a controversial feature at first and sparked a lot of uproar about privacy, I think that it has become one of Facebook's defining features. Above everything else, it allows for discovery. When I see an item that reads "23 of your friends have joined the group 'A tribute to those who passed at the Virginia Tech Shooting'," that has a big impact.
The Developer Platform: The big talk surrounding Facebook lately has been the introduction of their developer platform. Facebook has given developers some powerful tools to write applications that can be used within the site. While the inclusion of these third party applications takes away from my first point, simplicity, it more than makes up for it in increased functionality. Developers are just beginning to scratch the surface of what they can do. We are already seeing a few cool applications like Graffiti, which lets visitors to your profile draw you a picture. I think that, given some more time, we will start to see some really killer applications that greatly improve the overall experience on Facebook.
All of these factors put together make for a very appealing social networking experience that keeps users coming back. I expect that we will continue to see a tremendous amount of growth for Facebook in the coming months.