For instance, sites like livejournal, identi.ca and some others,provide a link to your profile data RDF ready formatted for FOAF.
On those sites, the RDF endpoints (FOAF) for my profiles there, can be referenced (and related to an object - Person tag within they are defined) inside the personal FOAF file (http://mmmattos.net/mmmattos.rdf) using OWL (The Web Ontology Language):
Unlikelym Facebook for instance, does not expose it´s data as foaf. But thanks to Matthew Rowe who developed Facebook FOAF Generator, you can extract its data and save it into a foaf file.
The great part here is that we are using FOAF to excercise the OWL relationships because there is a large amount of data already online. But this can relate any kind of information of any types.
Now using an RDF browser such as the one at OpenLinksw.com, and entering one of the FOAF addresses (http://mmmattos.net/mmmattos.rdf) to explore, navigate and aggregate all the data the LINKED data.
The next step is to make all those data inter-relationships to work as expected, which btw for is the key part to the success of RDF data modelling. For instance usage of rdf: resource where it should be rdf:about, and so on...