Earlier quoted context omitted.
There might be plenty of reasons why we don't know of such instances. For example, those being blackmailed might not know or have any proof that it's FBI who's blackmailing them. The FBI of course has endless opportunities of plausible deniability, starting with "rogue contractor". It all boils down to incentives: what exactly do you have to win from disclosing? What is the chance that people will believe you given t…
It's difficult for the FBI to use mass surveillance techniques to blackmail anybody. Sure they can make allegations, even true ones, but if they present evidence it would be pretty clear where it came from. Only so many entities have the ability to gather the kind of data necessary to separate fact from rumor. In terms of incentives, blackmail is generally a lower tier crime in terms of likelihood of success. Think a…
I'm not taking a concrete position on the question, but given the secrets that plain ordinary people keep, I would not be surprised if the data available on people is used for coercion and nobody talks about it.