They're using Google Trends to gauge how many people are using Facebook. Since 90% of this is people typing in "facebook" on their homepage to get to the FB login page, this might actually be a surprisingly good metric to track facebook usage.
Then again, the recent dip in searches could just mean that people are finally learning to use bookmarks, or click on Facebook directly now that most browsers show you your favorite sites when they start up.
Then again, the recent dip in searches could just mean that people are finally learning to use bookmarks, or click on Facebook directly now that most browsers show you your favorite sites when they start up.