PeopleAggregator has opened registration now [1]. One can ask other persons to establish a relationship. There is an XML-RPC service to ping for the latest FOAF updated on PeopleAggregator; REST and SOAP will follow.
XML-RPC:
Send a request to http://ping.peopleaggreagator.com/xmlrpc/ for the method foafUpdate.ping with a single argument, the URL of the location of the FOAF RDF. See the online tester for details.
[1] http://peopleaggregator.com
