Facebook's plugins use Open Graph, which is built on RDFa. It's RDFa that adds the property attribute to elements. Without this addition, plain HTML has no such attribute. (If you ask me, it's a strange design to add a new attribute without namespacing it, and to re-use half of a <meta> tag. But no-one did.)
To validate XHTML-with-RDFa, you'll need the DOCTYPE:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML+RDFa 1.0//EN" "http://www.w3.org/MarkUp/DTD/xhtml-rdfa-1.dtd">
This means you will have to be writing valid XHTML 1.1. More
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