It's the thing that's missing in most of these discussions - locks exist to keep out the polite. There's a reason that hunting up every bit of information publicly available about someone is called "stalking." It's considered rude, and is a social faux pas. Just because you can find out everything about someone doesn't mean you should and just because you've disclosed that information somewhere doesn't mean you should disclose it everywhere.
Disclosure should come at one's own pace. If you intrude upon someone's privacy and find out something they haven't told you and disclose that information to them, you steal their opportunity to tell you on their own time and in their own terms. And if it's an important thing and if you're been prying in their private documents or social media or whatever, you're going to break a trust that may not ever recover. Maybe sometimes it's warranted - like the cheating spouse who gets found out. But I think there are a lot of very personal experiences that someone can just not be ready to share yet, and when you pry to find out that information you totally rip from them the ability to tell you when they are comfortable with what's happened. I routinely take a week or more to socialize to people in my life when my significant other and I have broken up. (Historically, anyway.) I resent the idea that I am obligated to tell anyone anything. I tell someone something because I want them to know. No one is privileged to any knowledge of my life. (If I were married the situation would be different. When you enter into certain bonds and create a life together I think you kind of have to share certain things, especially as they impact your life together.) (This is all theoretical, this bit down here.)
And it's important that the level of vigilance reflects the risks of the situation. Casual meeting with a mutual acquaintance? Leave them their privacy. misadventures on match.com? Well... Suffice it to say that my first experience with online dating was a bust.. My second experience was with a girl who had a very attractive picture but was quite coy about personal details. Which made it a challenge. She didn't realize that calling from her father's land line gave me her last name. her email address gave me the school she was currently going to. First name, last name, local papers to scan gave me the revelation that she wasn't taking the year off to do an internship, she'd been put on academic probation for a year for, i shit you not, carving up her own neck with an X-Acto knife to give credo to the false rape charges she leveled against a crush who didn't want to date her. Not something that was likely to come up in casual discussion, but certainly something I was glad to know. I had the aforementioned Cosmic Bargain, though, so I went on the date. (a very well-documented date) Long story short - pictures were from high school, girl was from Hoth, ended up with the ugliest girl at the Spaghetti Factory talking about Ren Fair. Never even had to throw the "yeah, don't want to date you because of the rapey knifey thing" was able to leave it at "yeah, not giving you a ride to Renfair in Tukwila tomorrow. Or ever." Would I have had to disclose the fact that I'd stalked her secret at some point? Sure. And back when she was an attractive abstraction, I was quite conflicted about it. But since it never got that far, the faux pas was never practically committed.