Sadly it would be fairly trivial to do that if social networks standardized on something like FOAF; which would make it possible to just use SPARQL for queries. Of course figuring out how to deal with the privacy implications would be an impediment, but the basic querying would not be a big deal. Mapping identify across domains would also be a challenge without some sort of common identifier, or explicit mapping declaration(s), but it could be done.
Nobody (well, almost nobody) does it though. FOAFspace is practically a barren wasteland compared to what it could be. All the social networks hide whatever access they do allow, behind proprietary APIs at best. :-(