The etiquette for guests posting pictures of the wedding on social media?
Well you've gone to a lot of trouble to find a nice venue, to get made up, to send out invitations and you expect your guests to scrub up a bit in your honour. At the very least they are probably going to want photos of themselves with their nearest and dearest and it would be odd for them to not post these to social media. That is almost certainly what is going to happen unless you indicate something else.
You may have a perfectly valid reason for not wanting your wedding images shown on social media. If you tell me as a videographer that you do not want a highlights video to appear as public on Vimeo then I will respect that. If there is a particluar person who it is essential that their image does not appear online then again I will respect that too.
Obviously I cannot control what your guests do, (and it is unlikely that you can either) but its probably a good idea to decide in advance what you want to happen with guest posts. If your stance is that it is completely fine then its also a good idea to decide on an easy to recognise hashtag. Make it long enough to be meaningful and unique so it probably needs to be more than just your initials. Nicknames with the word wedding can be a good idea if you think they will be unique. Test out your hashtag by searching for it on social media platforms before you use it to check whether someone else has got there first and if necessary make it unique by adding the date of your wedding. Use capitals to differentiate between different parts of the hashtag for instance names.
I have seen weddings where the hashtag is written on a sign in the centre of the wedding breakfast room or you could also send the hashtag out on the wedding invitation as well.
Delaying the posting of photos to social media
I have also been to weddings where the couple have particularly asked guests not to post to social media until they have had a chance to view and select some of the offiicial photographer's posed shots. This is to ensure that the 'launch' of the official images isn't lost or watered down in the mass posting of guest photos.