Etiquette will have to adapt to advances in technology. So while I agree with you that it is still subject to social rules, people cannot just use PMs whenever they want to plan an event. If they did that then FB may as well not exist and we would just go back to simple e-mail. The purpose of FB is for everone to stay up to date on each others lives.
On the bolded, it's a 2 way street. In other words, technology shouldn't be used as an excuse to abandon long time accepted rules of polite behavior. Cell phones are a perfect example of this, and FB isn't far behind, imo.
Obviously, I'm on the side that says this is rude. And I say this because even if people aren't personally offended by "X", that doesn't mean "X" is ok. I'm sure most of us have been in that situation where a co-worker yakked nonstop about her natty nuptials, but we weren't offended because we had no interest in going anyway.
Boiling it down to its simplest components, etiquette is about doing what you can to make those around you feel comfortable/happy/whatever. If yapping about a wedding makes even one person feel hurt or excluded, I don't see how this could be anything other than an etiquette fail. I don't know LaciGirl at all, but I'm assuming she's not someone who goes out of her way to feel "offended" by everything. She seems genuinely hurt by this.
To me, this is like online dating
. Technology doesn't change the common sense aspects of it, nor does it change the etiquette. It's no more risky than meeting someone in a bar or at the grocery store. "Stranger danger" rules apply, and etiquette kicks in as to how you politely tell someone you'd rather be eviscerated than give out your home addy to a total stranger!
If FB wants to be the new norm for social interactions, that's fine, but I think it's even more imperative than in face-to-face situations to have clearcut rules of engagement. Pun intended