From Facebook Developer Wiki

Hi Can you please give a .NET sample for using fb_force_mode=fbml for iframe based application. For ex. my default page is default.aspx (based on CanvasIFrameBasePage base class of Facebook Toolkit of Microsoft). I use Iframe. But jjust for inviting people, I want to use fbml tags. My invite page (invite.aspx) is being called from default.aspx (IFrame) page with adding query string fb_force_mode=fbml . invite.aspx?fb_force_mode=fbml. But it returns that invite.aspx?fb_force_mode=fbml is not valid.

My fbml code is ;

<fb:fbml> <fb:request-form action="default.aspx" method="POST" invite="true" type="HangiDizi" content="Your text goes here. <fb:req-choice url='http://www.facebook.com/add.php?api_key=d43b5479ea3b7cc36f5c2bf479886366' label='Ekle' />">

<fb:multi-friend-selector showborder="false" actiontext="Invite your friends to use hangi dizi.">

</fb:request-form> </fb:fbml>

Any suggestion??

[edit] Good resource to have discussion about fb_force_mode in .net

http://www.codeplex.com/FacebookToolkit/Thread/View.aspx?ThreadId=20092