There are many factors on why Facebook is so popular at the moment. It became somewhat popular many years ago when it was first released to a handful of Ivy League colleges. Then it spread to most major universities, and then on to all the other places of higher educations. When it was getting stomped by MySpace it opened it's doors to high schoolers and eventually to the general public. Once anyone could start using it, everyone did. Instead of it creating a backlash to users who liked the exclusivity of it, it only created a fever that has yet to die down. Here are the things Facebook does to keep people happy with it.
Everyone's on it. Facebook reached a critical tipping point where it became wildly popular. It reached that point when it the number of people with a Facebook account, outnumbered the number of people without one. Nowadays, if you do not have a Facebook account you are pretty much a social pariah. The ability of Facebook to merge businesses and entrepreneurs in with individuals is something that other social networks like MySpace did not do well. On Facebook there's tons of commercial potential, and users do not seem to mind it.
No really annoying ads. Many of the ads on Facebook are repetitive, but they're not repulsive like the advertisements you see at MySpace. They're also targeted, so many times you will find things you really are interested, and this can lead to new hobbies and good and relevant finds.
It's all about the games and apps . Facebook has a wildly extensive library of addictive games and add-ons that keep people coming back to the site. Many of the social games available reward their players for spending lots of time online, or coming back online at regular times to complete game objectives. Many people will spend countless hours on these online games, and periodically check their News Feed to see if anything interesting is happening.
It plays to people's vanity. People like to talk about themselves, especially when they think they have an audience. Many people post many updates every day on how their day is going. If they have 100 or more friends they may feel that all of their friends are keeping up to date with them, when in fact chances are they've all hidden that person from their News Feed.
People are social animals. There will always be a social network available for people to use online. Right now it's Facebook. In two or there years it might be something new or it could still be Facebook. The future is unclear, but what is known is that Facebook has carved it's place in history as the preeminent social network at the time of this writing.