There is an interesting take on Zdnet about social protocols: "The XHTML Friends Network (XFN) microformat could eliminate the need for proprietary social networking services such as LinkedIn, Orkut, and Plaxo, if XFN was widely adopted among users and blog and Web presence authoring tools". It is definitely true that using all those social software tools require entering information in a wide laod of format (proprietary of course). Systems like XFN as they say might be useful (why don't they mention FOAF?) The ACM News Service offers a good summary of this discussion:

Currently, the proliferation of such social networking services (there are roughly 18 services) requires significant effort to continuously update personal profiles on each of those services. An ideal situation would entail individuals maintaining their own personal contact information on their Web site, blog, or contact management software, allowing all their friends to refer to that resource, and XFN allows for this type of nonproprietary social linking by adding relationship context to hyperlinks. The XFN profile describes possible relationship descriptors, such as brother, colleague, or even "crush." Blogrolls offer a good example of how XFN would help create loosely coupled social networks, but currently few blog authoring tools support XFN, such as by allowing people to use the rel hyperlink attribute that indicates XFN relationships. The hCard contact record standard uses XML to display contact information in a standard way, and could be used in conjunction with XFN and XFN-crawling software to automatically create directories of friends and friends' friends. This scenario would be helped by the incorporation of hCard and XFN technologies in Web presence tools--perhaps even Outlook--and blog authoring tools such as blogroll generators. Furthermore, the XFN concept could be extended to the business realm, perhaps in the form of XBN or XB2BN microformats

Personnally I juste use foaf and sometimes xfn but I would prefer to have just my adres book connected to a foaf file connected to my blog (to generate the blogroll) and other social systems. The difference might be that in most of the social soft, there must be an agreement by the two persons to create a connection; whereas in FOAF you can put whoever you want...