"English"[1] use this relationship mapping for gangs (mostly London and Manchester) and sometimes child sexual exploitation networks. But they're also interested in the content - see for example the Blackberry messaging used to convict people after the London riots.
I tend to be on the not bothered end of the spectrum[2] and I'm a bit wortied by the direction it's going in England with more stuff gathered, from more people, with less rigorous oversight.
[1] there are many different forces in England
[2] mostly because I saw regulation getting better. There are problems with RIPA, but it stoppwd some abuses.