When I first started playing in the Social Network Portability space I was working on some very light weight social network applications and wanted a way to interact with a global set of users quickly and easily. Right away I discovered foaf and xfn. I was immediately disenchanted with both technologies for what at the time I termed "statelessness", and in some late night discussion I thought about how my IM buddy list captured all the details I wanted from my friends networks, but allowed me to dynamically update it. From that came xmpp-psn, and numerous other projects that have all since failed horribly.
The state of SNP is not the rosy one that is given by the web community. Yes we have XFN, yes we have FOAF and XMPP, and DISO and SIOC. But most all of these technologies have massive flaws. That being said I think there's real hope for both XMPP and FOAF (more on why I'm leaving XFN off this list later).
So getting back to "Statelessness", XFN and FOAF both suffer from being static representations of a social network, both have the ability to describe complex graphs, be pulled real time, and as such aggregated. But the problems arise when a social network decides to make use of this data (say through google's Social Graph API), at this point the data leaves the control of the user and since it's stateless it becomes a "copy" of that data on the social networks network store, and in FOAF and XFN these technologies do not provide the means to communicate new changes back to the original source.
The current method of thinking seems to suggest that each time I join a new social network I'll point them to some canonical representation of my identity (a foaf file, an openID etc.) and the social network will then process the FOAF or XFN data, store the nodes and arcs in their database and connect me via that data to other users in their network. Then they in turn will publish that data. So the current model starts to fall apart at this point, as networks are then forced to aggregate that social data from each other in oder to stay relevant. This results in either having to be smart about caching and networking, or exponentially increasing number of polling connections (presuming we're using http). XFN further exasperates the difference between what's available for the Social Network provider and the canonical definition of the graph (by way of hrefs) as publishers really have almost no way of changing or removing content from that representation once it's been indexed, spread across multiple pages and distributed across the net.
Contrast this now with a technology like XMPP where the roster is stored on a particular server, queryable from any client in the network of federated XMPP clients, and can be adjusted realtime. This means that in order for social networks to stay up to date with XMPP they make a simple call to the users XMPP server and parse the results, and if they have changes they want to make, they can them communicate those changes back to the XMPP server. Looking at the traffic we now are back at a linearly increasing traffic graph, and the complexity of the operations has been reduced significantly. This being said, XMPP has it's own host of issues, not the least of which is it's ability to interact with the web over http, and the complexity of programing api's to communicate with it as it stands today.
It should be clear that the current state of FOAF and XFN aren't really going to cut it for todays social networks. They are great static publish side technologies, but that's not the world we live in with social networks. Social networks are wildly dynamic and they shift with context and need real time awareness. But but FOAF and XFN are doing something great, they are actually getting that data published and out there. Now we need to turn around and look at how those technologies can be made better for the creators of social networks.
So what do we do about it? Surprisingly completely on it's own and unrelated the rdf community has created some technology that I think might be extremely relevant here. It's called SPARQL and you can imagine it as being a simple SQL like query language for RDF stores. This it seems would make an incredible tool for defining the communication methods between hosts providing FOAF data. Now this problem isn't even close to being solved either, but the steps to solve it aren't particularly hard. There have been some interesting extensions to SPARQL that allow for inserts and updates. If we standardize on that as a means of communication then the supplier of FOAF data now needs to provide a means of digesting those SPARQL queries and announcing that they support updates to their FOAF file. Couple this with the work that Dan Brickley's been doing on oAuth bindings for FOAF and you have a powerful tool for not only representing your relationship data, but maintaining the state of those relationships close to realtime.
As for XFN?