Whew, sorry for the delay in getting back to you. We've been doing a bit of investigation, and think we've narrowed it down. Unfortunately, it seems to be an issue with Joomla itself, IE6-8 (but not IE9) and the Mootools Upgrade plugin. Basically, form validation fails with this combination.
We can't find other reports of this problem (are still looking), but you can test yourself and please let us know if you can recreate it. You'll need to go to the standard Joomla registration page at:
findsbook.com/index.php?option=com_user&task=register
However, your site appears to have a redirect setup to always go to the JomSocial registration. You'll need to disable that redirect (to test). Once you get to the standard Joomla registration page, try to just hit submit. For us (and we've tested on many sites), the fields turn red, but the form still submits. JFBConnect doesn't alter the normal Joomla registration at all, so if you see the issue there, it's not caused by JFBConnect at all.
We use the same validation scheme as Joomla, and in this particular configuration (IE6/7/8, Mootools Upgrade, Form Validation), it looks to be bugged. We are still investigating and plan to do more testing. If we find a solution, we'll gladly let you know what it is (and report it back to Joomla). We're not familiar with the validation Javascript code, so not sure we'll find the solution, but definitely want to. If we can't find something, we'll report it as a bug to Joomla.
Either way, please test and let us know what you find, and we'll definitely keep you posted as well.
Thanks,
Alex