From Facebook Developer Wiki

This feature is currently in beta.

To better understand how users are adding your application, Facebook has added support for a new URL parameter, called "app_ref," to the application add page. The idea is that you can specify a string identifier using this parameter, and Facebook will provide reports describing how many users added your application from each distinct identifier. This can give you insight into which links within your application lead to the most adds.

Usage

These links should look like the following:

• http://www.facebook.com/add.php?api_key=YOUR_API_KEY&app_ref=dash
• http://www.facebook.com/add.php?api_key=YOUR_API_KEY&app_ref=inv

The app_ref parameter is also supported on the about page (and will be passed along to the add page), as follows:

• http://www.facebook.com/apps/application.php?api_key=YOUR_API_KEY&app_ref=dash

In these examples, "dash" and "inv" are arbitrary strings - they can be anything (subject to the guidelines below).

Notes

• The values for app_ref must consist of only alphanumeric characters and underscores, and be at most 8 characters long. Values not meeting these criteria will be ignored.
• The plan is to report statistics on the existing application stats page, but it will probably take a little while before you start to see these.