mixi launched in February 2004, and overtook competitor GREE as Japan's top SNS within its first year of operations. Now the service has 12 million users, and a very solid 60% of these have logged in in the last 3 days.
He gives 4 reasons he sees as responsible for this success:
1) People needed a service that reflected their real offline relationships (emphasised over virtual ones through privacy features)
2) The prominence of Consumer Generated Media began to rise
3) Benefits of the network effect
4) Dynamic content
It is the 4th reason that he believes saw mixi move past GREE. The focus was always on a "Communication Enabled" SNS. Adding features like the Diary and Footsteps (where you can see who has visited your page) increased repeat visits and made mixi part of daily life for millions of people.
I would add that offshore players seem to be missing the boat here big time ... on the one hand by not localising their interfaces, but more critically ignoring the massive Japanese mobile market.
Later in the article was the very interesting announcement that mixi will be implementing the OpenSocial API. To date mixi has been very conservative in adding new functionality, considering the influx of cash with their public float. The had a minor site redesign, and added some very basic CGM features. OpenSocial will provide an entry point for a huge amount of functionality, but as they say - the devil will be in the implementation.