Facebook is useful for finding long-lost friends and acquaintances. It's somewhat useful for assembling groups of people (e.g. likers of Facebook fan pages) interested in a topic, and distributing information to them. It's a good way to distribute images whose copyright is owned by someone else, since the lack of search ability by the general public makes it hard for the copyright owners to catch you (email also has this advantage). It also apparently has algorithms that reduce how often you're exposed to the same content (e.g. images) posted and re-posted by many different users. Who knows what else its proprietary algorithms do (or how they do it) to help users find stuff that they're likely to be interested in, and weed out what they are unlikely to be interested in.
Facebook is designed to be an easy way for procrastinators, or those who have a few idle minutes to fill, to find stimulating material. There is always something new and entertaining on the feed, and it's coming from a multitude of diverse sources, rather than being monotonous. One does not need to ponder what site to surf; one just goes to Facebook and is presented with whatever content has most immediately been posted. It's much like surfing TV instead of browsing a magazine rack or bookcase.
Likewise, it is designed to provide instant gratification to those who want to be heard. You can post a status and get immediate responses. The site is designed to bias these responses positively (since there is a "like" feature but not a "dislike" feature), fueling the crack-like addictiveness of this means of obtaining positive feedback.[3] The "share" feature reduces the workload involved in re-posting content and makes people think, "Wow, I'm really disseminating my content quickly and virtually!"
The fact that random people can't post messages, but have to be added as friends first, is a prophylactic against the kinds of vandalism, spam, and other misbehavior that requires people to vigilantly play the role of moderator on wikis and blogs. Facebook lets each person have a venue for personal expression, without requiring him to set up, maintain and publicize a website. In fact, friends will search for and come to him. It can work synergistically with one's personal website; one can post status updates and create fan pages linking to the website, and one can post on the website links to Facebook profiles, fan pages, etc.