My son and daughter were both into this game for about a year before they lost interest and decided they would rather save their money or spend it on other things.

I think the thing they liked the most at the start was the fact that there were a relatively large amount of things they could do and discover and then share and showoff.

But after they played for a while I noticed they stopped wanting to show me neat new stuff in the game and instead tell me about the people they were playing with. I was really surprised when I learned that my then 13 year old son was regularly spending hours online with armed service personnel stationed in Europe, disabled diabetics from Iowa in their 40's, thirty-something gay couples from Palo Alto, off-duty nurses from Los Angeles, etc. In short just about every kind of person he would never otherwise consider part of his social group.

So personally I think the real reason it is successful is that it seems to be able to create the same kind of opportunity for shared experience across cultures in much the same way early radio and television did.