Subpoenas need some probable cause to be issued. It'd be interesting to estimate what percentage of target population you'd need to subpoena complete details on in order to build a social graph of the whole population, but I feel that that volume of legal activity would create waves even if it were only around 1%. Most court systems are overburdended as is, a significant increase would be noticeable.

Of course, if you were just getting that information as a matter of course in many arrests and were willing to accumulate it slowly over time it's very possible. I'm preoccupied with scraping issues as that's my only available avenue.