try this:
Add
runat="server"
attribute to your iframe.
<iframe id="urIframe" runat="server"></iframe>
Once you add
runat="server"
attrubute to your iframe you can access your iframe from codebehind.
Now add
src
attribute from codebehind:
urIframe.Attributes.Add("src","http://www.facebook.com");
Good luck.