We don't really have a Facebook plugin in mojoPortal, you just embed their widget as html and script so if there is a problem with their widget in IE 9 it will probably be up to Facebook to fix it.
We do have a facebook like button, if it doe snot work in the final release of IE 9 I will look into it but that is really just also using a widget that facebook prescribes so we still may depend on them to fix it if it has problems with IE 9.
I have not installed any RC of IE 9 and don't plan to install it or try it until it is released in its final version.
Best,
Joe