Coding Errors

Simon,
I tried only on the home page, I'm assuming the errors are consistent throughout, but if not, let me know.

On the home page, the main errors come from:
* The XMLNS tags at the top. We add these for Facebook integration to specify the tags that Facebook supports. Facebook, unfortunately, follows the RDF1a (I think that's what it's called) standard, and it's not completely compatible with HTML5 validation. However, every browser supports the tags and they are important for markup purposes of the tags.
* The og:title (and any other og: tags) again are from Facebook. They set the Open Graph tags for the page and are defined in the XMLNS document above.
* The g:plusone tag is a custom tag from Google for their G+ button. Again, they don't conform 100% to spec, but all browsers accept the tags.
* The other issues in there don't seem related to JFBConnect.

In general, validation shouldn't be a critical priority. Browsers all tolerate the invalid specs mentioned above as they're so common. Additionally, Google has come outright and said that validation plays no part in SEO. That's not to say it's something you shouldn't try to fix where possible, but the issues above can only be fixed by removing those features.

I hope that helps explain,
Alex