Hours after VentureBeat kicked off the latest round of social search speculations, Google announced the Social Graph API. The API could use information contained in files and links on a site (including FOAF and the XFN relationships meta data, which you may already be using) to identify social relationships between the people behind websites.
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.)