We've done some preliminary testing with JomSocial 2.4RC2, and things have been positive so far.
JFBConnect doesn't insert it's own Login With Facebook button into the home page of JomSocial. You may have manually done that in their template before, and the upgrade simply wiped out that change. Try taking a look at the page below and see if you can re-add the button. The documentation is still for JS 2.2, but should be pretty accurate for 2.4:
www.sourcecoast.com/jfbconnect/docs/thir...ration-for-jomsocial
Hope that helps,
Alex