Yes, there is social media etiquette that extends to weddings. It is quite common to see guests snapping pictures and posting them as fast as their fingers will allow, and some couples encourage this. You have probably seen a sign like the one below at a wedding you’ve been to recently.

Bear in mind however that not all couples are open to having their wedding displayed on social media. Some will explicitly advise you through signs such as the one below.
If you are not certain of the couple’s feelings towards sharing their big day on social media, err on the side of caution and do not post photos. It is quite possible that the bride and groom want to be the first to share photos.
Here are some additional tips:
Do not post photos of the couple before they even walk down the aisle. The big reveal, especially for the bride is a big moment. Don’t ruin that. Imagine a groom scrolling through Facebook and coming across a photo of his bride-to-be in her hotel room wearing her wedding dress.
Do not get in the way of the professional photographers. No matter how much you pride yourself on the excellent quality of your photographs, YOU WERE NOT HIRED BY THE COUPLE TO TAKE PHOTOS. They hired a professional to do this and you should exercise courtesy and stay out of the way. The photographer should not be in a competition with you to get a good shot.
Don’t post an unflattering photo of the bride (or groom). No explanation necessary here…
Do not be so glued to your phone that you become a ‘host’ for the event, giving commentary through posts to social media every couple of minutes. It is for this reason that you will see signs such as this one. Remember that you are there to celebrate with the couple.
Brides and grooms, these last two tips are for you. For starters, share the news of your engagement with your close family and friends before posting it on Facebook. It would be insensitive to have your cousin who saw your post bring up your engagement to your mother who is totally unaware that her precious daughter has been proposed to. Secondly, do not take to social media to gripe about the wedding planning. If you find there is a problem with an invited guest or a vendor, speak to the person directly, versus airing your frustration for the world to see. This will only create further problems.