My faux-pas radar went off when I saw a friend announce the birth of her sister's child on Facebook. Maybe it was a moment of overzealousness, I justified, but an hour later she came back to tell us the name. For all I know she had permission to make the announcements, but this — while the most overt — is hardly the first Facebook spoiler I've seen.
The backdoor spoilers are probably the most popular. Congratulations when there's no announcement or pictures of the bride before she walks down the aisle. It may be innocent enough, but I think it's time to talk about an unspoken rule: don't talk about it online until I do.