the way these social software sites aught'a be: run a local content daemon and host my own data. sites like multiply could still exist but they would be aggregators for people' content. another option is a local application could be written that would be free of the limitations of a web-based app. Searches would be gnutella-like flood queries (with optimizations of course). Your own policies for who sees what and when can be defined and such a service would exist on networks isolated from the Internet since no central server is used.
some people would host content for multiple people when they do not have the hosting resources of their own. instead of a local daemon, apache+webdav could provide controlled access to data, but it couldnt do complex searches. Maybe we should all just setup mysql with a guest account and a standard set of database/table names.
OS X's sherlock seems to be in this arena, a local app with modules that are auto-discovered. The movie search app is really useful.
plink.org is a FOAF aggregation site that is a interesting idea, but terrible execution.