Facebook is deprecating FBML and recommending JavaScript SDK (on Graph API) with Social Plugins to implement features inside an iFrame app.
If FBML goes away so does its controls including the tabs FBML. Excerpt:
fb:tabs
Advanced Topics › Legacy FBML › fb:tabs
Renders a group of standard Facebook navigation tabs. Must contain at least one fb:tab-item.
Is there a Facebook replacement for the tabs UI control, or is it now left up to the developer to find their own tab control for use in iFrame apps?
Another potential avenue is XFBML - I think if Facebook is deprecating FBML then they might be deprecating XFBML by inference, but I cannot find a definitive statement about this. FBML might refer to the type of app, or it might refer to the tags because FBML tags can be used inside an iFrame (i.e. usedoutside of an official FBML application). I would choose to use XFBML in an iFrame app if it's not being deprecated with standard FBML.
Apparently you have to find your own tab ui technology (same for other specialized controls offered by FBML). It's best left up to the programming technology used in your iFrame pages. e.g. PHP, ASP.NET, jQuery, etc.