Paul,
Still not sure, honestly. A white page is almost always a PHP error. Either way, for both of those plugins, simply enabling them doesn't perform any cURL operations.. we don't try to fetch things from Facebook on every page load, or that would be a big burden on your site.
Can you try the following:
* Disable all other JFBConnect plugins, especially the JFBC Profile plugins
* Go to the JFBConnect Configuration area.
* Hit "Apply" (even if you don't change any settings). This will refresh our configuration settings in case some bad value was written.
* Re-enable the JFBCSystem, then authentication, then user plugins
* If that works, enable the Content plugin
* If that works, enable any JFBCProfile plugins you need.
If it breaks with a white page along the way, my guess is that your server is not allowing Joomla to overrwrite it's error display settings (preventing the error from showing). Do you know where the error logs for your site are located? Any PHP errors should also be logged, its generally just easier to display them though. That error will really help.
However, you can also try to simply re-install JFBConnect. Just use the Installer and do it again like it's brand new. It will refresh all the files, but leave your configuration settings intact. Then, go through the same steps above.
Really hope something there gets you going!
Alex