Re: So before Snowden...
Surely pre-pay phones and fairly innocuous chatter is going to be the best bet here.
Look at the legitimate (i.e. non-obfuscated) chatter going over the web on an average day, facebook,stack overflow, twitter, amazon etc.
How really can you determine "hello mate, pint and same again please" -> "sure, same place ?" -> "yes, bit later - say five?" -> "see you there ;)" relates to some illegal exchange of goods or services.
It's fantasy - the blatant point of all the trawling is to provide an ability to *retrospectively* throw every possible extra charge at someone under arrest to get them to give up, whomever they're associated with.
I'd be willing to bet that the big scale stuff is disguised as company purchases for intangibles (IP payments perhaps) and looks like any other sales ledger unless a forensic accountant gets her hands on the books.
The little stuff is just not hidden.
I seriously doubt a sophisticated infrastructure is required to work out what "got any weed?" -> "how much do you want?" could possibly relate to.