I've been working recently on some new attempts at understanding what exactly is the best way to model Identity Attributes, and who should or can state them about a user (Alice from here on out). What I've concluded is that anyone can assert attributes about Alice (truthful or not). Sometimes you can find these assertions, and sometimes you cannot.
There are at least two separate layers for attributes: a horizontal and a vertical. The horizontal layer represents what can be best described as Alice's vCard. This layer represents attributes which share commonality across almost any application domain, be it social or otherwise. Other tidbits like personal preferences, the type of cell phone she carries, her iTunes playlists, and whatnot can also be lumped in here (tho in some cases, slices of these end up also in the second layer. more on that in a bit).
The second 'layer' is comprised of vertical pillars of domain specific attributes. Financial, social, business, Alice's Rock Climbing Club, her neighborhood, etc... these carry both attributes asserted by others, as well as Alice's own attribute statements.
Reputation systems (attributes asserted by others of Alice) tend to focus only in these vertical domains. Reputation systems cannot effectively function as a horizontal layer, as the perspective of Alice varies from domain to domain. But proximate domains may borrow reputation and other attributes from one another, tempered and weighted by their proximity to one another.
So the relevance of Alice's attributes is proportionate to the domains “distance”, and tempered by the asserting parties reputation in both domains.
This introduces the notion of the “over-layer” domain, where arbitrary aggregations of attributes from nearby domains define a third dimension. This domain, rather than being just another vertical domain, is my persona to a given community.
I've been dabbling with a variation to Paul Madsen's Vector Addition for Identifiers, where the vectors are influenced by these sorts of behaviors between and across domains. Hope to have that RSN ;-)