This is not a passing fad. There are a number of other online services and virtual worlds that are growing in popularity. There are indications that Google and Microsoft may also be releasing their own virtual worlds to rival Second Life. And it is reported that $1 billion has been spent on the development of 35 different virtual worlds in just the past year. Additionally, $15 million has been spent on advertising within these programs in 2006, with this figure projected to be $150 million by 2012.
The author of this article talks about why people are motivated to spend hours behind a computer screen-- and it really just comes down to the fact that the computer screen offers an instant "comfort zone". It's also a way for people to essentially run away from their problems, albeit temporary.
People are literally leaving reality behind and living in a fantasy world.