That being said, I don't believe you can expect an enthused bride with an 8-month engagement to never mention it on FBook.
I do. I have been engaged since August and my wedding is next October. I changed my status when I got engaged and due to equests from that posted a pic of my ring. And that has been it, publicly.
Have I sent PM's via FB? Yes of course. Have I posted vague updates "spent the evening at an expo with Julie - 10 pounds of gift bag to be lugged home on the subway!" where I didn't say it was a
bridal expo, and it was open to the public anyway and people know I go to all sorts of shopping vents, promotional events, etc (and happily bring along anyone who says "hey next time let me know I'd like to go)? Yes. Or I might post a vague "it was good seeing you!" on my FMIL's page after a day spent wedding planning. But I don't mention anything that is clearly identifiable as "wedding" and I expect other's do behave similarly (and IME, most - especially the one's with couth in general - do).
I was a complete person with a full life before becoming a "bride to be" and found things to post about, those things are still in my life now. The wedding is not an event everyone is invited to, so I follow the basic ettquette rule of not posting it where everyone would see it. I don't talk about my wedding at parties, or at work, or at the bar either, unless asked or in a private conversation with a friend who is invited - in conversation with casual friends I find other topics to talk about or remain vague (for example, if asked about any plans this weekend? "Oh just some shopping").
Basically I treat FB like party conversation - mentioning your recent vacation, or a new restaurant that others still have the opportunity to experience on their own is great, gushing about a party you are hosting but not inviting everyone to, is not.