My first full time job (early 2000s) was working for a firm that did online cybersecurity related investigations for Fortune 500 companies (generally via a 3rd party law firm they had retained).
A big part of this was running investigations into people running "pump and dump" stock schemes on Yahoo message boards. We would generally start by scraping all of the posts for a user who had instigated one of these and then handing off the posts to an analyst.
It's amazing:
a. how much info people give out even when they think they are being careful
b. related to a, how even small tidbits combined over time can build a pretty accurate picture of who someone is.
e.g. they post "oh man, the Cubs lost", then a year later "went for a walk on Lakeshore drive", another year later, there was a fire at my local subway stop etc etc and you pretty quickly narrow down the rough neighborhood where they live in Chicago.
Combined with tools like Lexis Nexus and you get a list of people that you can narrow down by age, sex, occupation etc and we could narrow it down to <20 people based on other info they had shared.
Then you fold in their posting patterns and it's pretty obvious who is at work (posting 9 to 5pm) vs home (posting 7pm to 1am).
Again, you keep adding constraints and the intersection of the Venn diagrams gets smaller and smaller.
This was all in the early 2000s before we had cellphones that tracked your location and ad infrastructure that followed you around the internet.