I have the real name of several so called 'anonymous' online personalities, but I won't divulge this info. I was curious recently about a Twitter account posting under a pseudonym and wanted to see if their opsec was tight. Turns out it wasn't. Their real name was discovered in ~15 minutes with some heavy Googling.
Imagine if you can simply throw money at the problem and forgo Google entirely, getting not only their legal name, but other PII too?
"how much they unwittingly divulged to various services (services that sell their data to people-search firms and other brokers)."
Would a regular person have unwittingly given information to these services?