Why do you think Facebook has become very popular?

I've noticed that Japanese Facebook users have been increasing recently. I suppose that the remarkable role Facebook played in the civil disobedience in the Middle East and the popularity of the American movie "The Social Network" have been helping Facebook become more popular in Japan. On top of that, I've heard that Facebook has been very serious about expanding in the Japanese market in the past year in order to increase Japanese subscribers. It has been said that the immense popularity of Japanese social network service "Mixi" has been forcing Facebook to struggle to win the hearts and minds of Japanese people.

Speaking of social network services, more than five years ago, some friends of mine living outside Japan invited me to join "Hi5" and "Tagged". After that, they urged me to use Facebook, as well. Since then, I've been on Facebook for five years. On the other hand, I haven't logged into both Hi5 and Tagged for a few years because I have no active friends there.

I'm wondering if Facebook is more popular than the other two. If so, why do you think Facebook has become very popular in the past few years? What is the biggest difference between Facebook and the other two ?