Designed for humans first and machines second, microformats are a set of simple, open data formats built upon existing and widely adopted standards. Instead of throwing away what works today, microformats intend to solve simpler problems first by adapting to current behaviors and usage patterns (e.g. XHTML, blogging).Microformats leverage existing XHTML tags such as address, cite, and blockquote and attributes such as rel, rev, and title to create semantically meaningful information. Essentially, profiles of XHTML for encoding particular slices of identity (e.g. personal info, calendar data, etc) as HTML metadata.
There is a microformat proposal for expressing social identity, it's called XHTML Friends Network (XFN). In XFN, the social connections between users manifest themselves as hyper-links, just like connections between pages. Such an XFN link is distinguished from a normal link through its use of the rel attribute on the HTML <a> element. For instance, a user named Bob could represent the nature of their relationships to Josh, Kat, and Mary with the following elements embedded somewhere in their personal page (e.g. a blog).
<a href="http://josh.example.com/" rel="friend met">Josh</a>
<a href="http://kat.example.com/" rel="met acquaintance">Kat</a>
<a href="http://mary.example.com/" rel="co-worker friend met">Mary</a>
Each of the three could have a similar set of links in their pages, perhaps including a link back to Bob to represent a bi-lateral relationship. Depending on your point of view, the ability to indicate a relationship to somebody else without their permission or action is either a good thing or a bad thing. One could, for instance, indicate a totally unrequited romantic relationship.
XFN defines a short list of allowed values for the rel attribute, things like met, colleague, sibling, etc. For some applications this level of granularity for describing relationships might be sufficient - but not for many others (because ultimately the social data has to enable something or else it's simply an ego trip). To be fair I expect this was a deliberate trade-off in favour of simplicity over flexibility.
The alternative is to not attempt to prescribe what are valid relationship types but rather ensure that users can specify their own. The Liberty Alliance's People Service defines two such mechanisms in support of this model, groups and tags. Groups allow a user to define collections of the base members of their people service, the tag element can be used to add additional metadata to both members and groups. For instance, I might define a 'Identity Bloggers' group to carry those individuals who I wished to be able to leave comments on this blog. For other users who didn't quite fit this grouping, but for whom I still wished to enable commenting, I could define a 'respected' tag and apply it to each such member.
There are of course bigger issues dividing the microformats and XML protocols camps. Microformat advocates preach 'pave the cowpaths'. Sure, if the cows have been going to interesting places.