The chief measuring stick for Facebook Platform applications is supposed to be the number of “active users.” While an “active user” is vaguely defined, it roughly means the number of users who touch the canvas page, play a flash file, or otherwise interact with an application in a day. It’s a tall order to have high daily numbers.

Facebook, of course, has always reported the number of “active users,” not total users. That’s a trend-setting choice. But what exactly is an active user for Facebook? Apparently, it’s not the same as it is for application developers.

The first section of Facebook’s “statistics” page boasts 42 million active users:

• More than 42 million active users
• More than 200,000 new registrations per day since Jan. 2007
• An average of 3% weekly growth since Jan. 2007
• Active users have doubled since Facebook expanded registration in Sept. 2006

But the second says that “half of active users return daily.”

• Sixth-most trafficked site in the United States (comScore)
• More than 54 billion page views per month
• More than half of active users return daily
• People spend an average of 20 minutes on the site daily (comScore)

So what is an active user? For application developers, it’s daily users. For Facebook, it appears it’s something else. The company has the right and interest in keeping this metric vague, but it can be a little confusing when on the Facebook measuring stick, an “inch” means different things in different places.