This week I lost another kid to Facebook. Like most Millennials, my 18-year old daughter has been addicted for years, but my son, who just turned 15, had so far found it uncompelling. No longer. This week, he went from just a handful of friends to over 140 friends. Like his sister, he now spends hours uploading photos, posting remarks on friends’ walls and making new connections.

According to Gallup poll data, there are dramatic differences in amount of time spent on the internet by age group. Those spending at least one hour a day drops steadily with each successive age group, from 62% for Millennials to 24% for their grandparents. (See chart) If my kids are any indication, the ‘Facebook Effect’ is a large part of the difference. (My dad likes his computer, but doesn’t know what else to do once he’s done with email).

According to Nick Burcher, who follows Facebook and other social media site usage, Facebook’s U.S. user base grew from 27.8 million users in July to 42.0 million in December, a 51% jump. Internationally, its user base grew even faster. Altogether, Facebook added an eye popping total of 47 million new users in 2008. The largest non-US user bases are in the English speaking Canada and UK. (For more on international use of Facebook relative to Twitter, see Brendan Mitchell’s blog post.)

There is no doubt that Facebook dominates the lives of many Millennials, but whether this pattern will continue indefinitely is still up for debate. In our research among young adults we are starting to hear some reservations about Facebook as the end all be all social network application. While Twitter is not really registering yet among young professionals we talk with, we are hearing that they think of Linked In for professional use, and fear that their Facebook profiles may come to haunt them as they advance in their lives and careers.