My generation (40s) I received a written thank you for every wedding we went to / sent a gift for. However, recently, showing my age, we have gone as friends of the mother of the bride (3 times, 2 families, mother widowed). In the "less posh" family we got a lovely card with a photo of the bride and groom, a couple of sentences about the day and thanks for the gift. In the "very posh and always a stickler for doing the right thing family" - nothing from either wedding. We sent a gift for one wedding which we could not attend as we were away and sent it 3 weeks after the wedding - some silk from India where we had been on holiday which we had arranged to be woven to a specific colour scheme - nothing.
I actually think not saying thank you is really, really rude. I write to thank people for dinner, for any gifts, for parties, weddings etc and am ensuring my children do the same. Yes, it may be by text, email, Facebook or post (love the post) but it is good manners.
I think even large weddings (and ours was 200 people) it doesn't take long to write a few lines to say thanks for the 100+ presents we had and to thank people for the effort of turning up often at considerable expense. Also, it is usual to send a gift if you can't attend and, in this case, there is not always the option of verbal thanks.
What I really, really, really hate, however, is the grabby table of gifts on display. My worst wedding (actually, no thanks for that one) was when we all had to return the next day to "watch and admire" as the bride opened the gifts. Like a really bad Christmas where you get nothing......the groom's sister kept muttering about how "common" it was! Our gift (in a card) was ignored as well.
Rant over.