I've read a ton of stuff saying that this basically doesn't apply to Millennials, which is causing huge problems for employers that like to hide compensation from employees.
This is really interesting (I'm a Millennial). I shared my salary with my closest friends (and basically anyone who asked, because I'm way open about money), and I know what my closest friends made/make. I recently met up with an old coworker when he was in town, and it was totally not awkward to ask what he's making now (a couple years after I left - we started in the same role). I'll be job-searching soon and I'm pretty sure I'll be talking to friends about specifics of their salaries. I also think that it's easier to talk about salaries earlier in your career - there's less disparity in incomes at the start, than there is after years of performance reviews/promotions/compensation evaluations, which might leave two people who began with the same salary/job in totally different positions (and feeling badly about exposing that).
As to the general question, let me offer you a story. I've commented on another website, with a username that's a nickname some friends use for me. My SIL somehow found my account (she knows the nickname and used it to search for something I posted online), and found a years-old comment that mentioned a bad experience that I had with a situation that involved her (but the comment was all about my feelings, no specifics on her and anyone else involved). And it was a bad situation, which left me feeling unhappy and oddly violated in a way. Nothing I wrote was not shared in person earlier with those involved in the situation, and the focus of the comment was on me (not on criticizing others). I wasn't ashamed or embarrassed by what I wrote, but some of my in-laws got really upset because they felt it wasn't appropriate. Won't lie, the situation did shake me, I've never had someone cause a commotion about anything I've written online. It was the only comment (of dozens) on that site that mentioned my SIL, I didn't even remember writing it (when shit hit the fan), and I don't feel I was in the wrong in any way. I never commented on that public website with the assumption of anonymity ... but I was really unsettled by the situation. I'm not sure how I feel about the whole thing (mostly angry), but I guess it's a good tale to get you thinking about possible ramifications of posting identifiable information.
I still share everything about my finances on this site, maybe that will change when I have a high net worth. Anyway, it's undeniably true that over the course of lots of posts, tons of personal information gets revealed, and it really adds up (if you're not cognizant of/thoughtful about what you're sharing). People who know you can probably identify you. That doesn't stop me from posting - but I do think before the incident above, I was a bit naive about how sheltered my comments on various websites were from my real life (turns out they're not, not the way I was doing things).