Like other meta data, this information is not generally available to visitors to your website (unless they view your source code). However, Google is quick to note that this data is already public and clarify that they will not be using private profiles or social networks to find relationships between people.
The XFN meta data can indicate relationships from everything to casual acquaintances to familial relationships to crushes. (But, uh, you might want to be careful about publishing that kind of info on your website.)
If you use WordPress, you may already be producing some links which include the XFN relationships meta data, specifically in your blogroll. When adding a link, below the dialog box for the URL et al. there’s a Link Relationship (XFN) menu (which is probably minimized):
By checking boxes or radio buttons, the rel: field is automatically filled in with the appropriate attributes, for example:
This would generate the code in bold below:
<a href="http://www.example.com/" alt="quite exemplary" rel="friend neighbor parent">Example Website</a>
The simple reason why this capability has long been included in WordPress is that WordPress founding developer Matt Mullenweg was also among the authors of the XFN relationship meta data.
Now Google will begin using this data to establish and interpret not just linking relationships between sites, but the interpersonal relationships of site creators, including multiple sites authored by the same person.
Naturally, this is more germane to personal sites, where there is a single person behind a website who can have interpersonal relationships. (At least in my experience, companies generally don’t ‘date’ or have ‘crushes’.)
This of course makes us wonder what Google wants with this information. Will they begin promoting results to me based on who my friends are? Somehow, I just don’t think the people I’ve linked to on my personal website and identified as friends are looking for or [bureau of economic analysis] anyway.
But even if they were, why should Google assume that the sites that my friends in Ohio and Alaska find relevant for anything from weather to news to vacations would automatically be relevant to me?
For more information about the API, check out the Social Graph API. See also the full profile of the XFN meta data, including the valid values for the attribute, as well as coverage from Search Engine Land, ReadWriteWeb and Google Blogoscoped.