Herman,
If you haven't already, please make sure you've read our
Facebook for JomSocial
support guide. That has the initial steps you'll need for compatibility between the two.
For the post to Facebook on Registration or Login, please try using just a Message at first. Make sure that works. Then, try adding a URL link and an image link, if you want to use those. When you add them, make sure you are using a *full* url, including http://. If those URLs are incorrect, the post will not be accepted by Facebook.
Beyond that, can you let us know specifically what features you're looking to get working and how it is failing?
A link to your site will also help with diagnosing specific issues.
Thanks,
Alex