It's probably always been dead to 16-18 year olds because the rest of us are there -- and 16-18 years is a tiny demographic.
I am one of those people who hate Facebook but still use it because people are on it, and it seems to be the default way to keep in touch with friends and family members, when you don't really have anything to say.
Adults used to write letters (kids still do), and sometimes put pictures in those envelopes. I haven't done that in decades, it's just easier to type one sentence instead of write a whole page, and post the picture there, and it is 1:Many communication, so there is a chance someone will care.
I tried Whatsapp and wasn't impressed. You can only use it with people who know your phone number anyway, and if you travel outside your home country and have to put in a new SIM card for a different phone number, you're offlne.