Do you mean you post an update on your facebook page and it become autopost on your xoops site automatically or you make some update content at your xoops site then you need automatic autopost to your facebook fanpage?
I dont understand why XHTML validation so important for you even google have 24 Errors, 3 warning(s) http://validator.w3.org/check?uri=google.com&charset=%28detect+automatically%29&doctype=Inline&group=0
facebook also 46 Errors. http://validator.w3.org/check?uri=facebook.com&charset=%28detect+automatically%29&doctype=Inline&group=0
valid W3C Compliance not improve seo score over 1 %.