I have about 300 friends on facebook. If I were to get married and have a "big" wedding, an all-out bash, only about 1/8-1/3 of those people would be on my invite list. At least 200 of them are professional contacts or old high school pals or people I met twice or eHellions or ex-boyfriends or what have you. No one I don't like, but definitely not people I would invite to my wedding. An intimate wedding might include 15 of the.
An immediate-family-only wedding would include 3 m (and that includes my BF, who I'd be marrying in this hypothetical), or 1% of my friends list.
It's not those people who I'd be worried about, though. They really have no reasonable expectation of an invitation.
It's the second cousin whose wedding I attended 5 years ago, and we haven't really been in much contact since, or the work colleague I get along well with at work, and we've socialised a few times, but I'm not inviting any work colleagues, or the friend who thinks we're a little closer than we actually are, or as in the OP, the aunt who is not being invited in a family in which aunts generally are invited to weddings; all of whom might feel like they might warrant an invitation, and would be hurt to discover that they aren't getting one. Those are the people I wouldn't want seeing the posts like, "Mailed the invitations today!" Bonus hurtful points if not receiving an invitation in the mail a week later is the first they know they're not invited.
I am not going to censor myself or my excitement over personal events on the off chance that one of those folks would be upset about not being included in such an occasion.
But would you not censor yourself if you happened to be standing at a party with some of your intended guests and some of the types of people I've mentioned? Isn't it against ettiquette to talk about an event you're hosting in front of people who have a reasonable expectation of an invitation, but aren't getting one? How is FB different?
I'm using your post, jmarvellous, but am responding to many posts. Sorry to pick on you.