FOAFers complain XFN is half-baked. And that it confuses pages with people. A URL is apparently insufficient to be a UID for someone. Though Ryan appropriately responds that
Second, why can’t URLs be used to identify people? Homepage URLs are as meaningful as email addresses (or hashes of email addresses, as I believe FOAF uses), so why can’t they be used to identify people?
Danny disagrees :
What happens if someone made a statement about the URL, say providing its dc:creator?
The use of a URI is questionable for the purpose in any case, see Identifying things in FOAF.
The first of these would certainly create confusion, and there's no type-system in XFN to stop it. The second point I'm not sure I get so I'll reserve judgement.
Ryan's other big point : Microformats are designed for "humans first, machines second".
That seems important. Any format with two (potential) user constituencies is probably in better shape than a format with only.
Danny is being a bit disingenuous about this by saying
XFN markup isn’t really any more human-friendly than FOAF’s RDF/XML, and what’s more without a machine to interpret the stuff there’s not much for the human either way.