I rather meant, if a significant bunch of people use this service for illegal things still being out of prison, hence you can consider this "metric" too (along with others) whether Telegram is secure. However, thre is no statistic for "being out of prison" people who used Telegram like this.
This particular surveillance and espionage tool might be far too valuable to employ it against some petty criminals. It is known that a lot of Russian dissidents and members of the opposition use Telegram - so covertly spying on them might be much more important for FSB directorate.