Those are hard times. Facebook discontinued support for Notifications.send method March 1, 2010. Calling this method returns error code 3 (Unknown method). Instead, developers are directed to use other communication channels. From those some are not yet even a part of facebook-actionscript-api yet (dashboard) other not published from facebook (invites) and those that may work via api are experimental and may change any day (activities).
Excluding streams as communication channel, the only usable way, these days, to acquire some new facebook application users is using <Fb:request-form> (part of FBML). When your application is IFrame, you may find interesting that you are able to use FBML within your HTML files. In order to make this happen, you have to use <Fb:serverFbml> (renders the FBML on a Facebook server inside an iframe). Now, lets see how to open request form in your Facebook Iframe application: