This is an honest question...
How is a promoter supposed to promote a show and sell tickets if the internet can't? I mean... People into an artist are somehow linked to them either by Facebook, Twitter, or their official website. So they know about the show.
Do people really look at a telephone pole and decide to go to a show these days?
i by no means know everything...so this is only a partial answer
but i do know that the people who are connected are the core audience. Those are the people that are coming for sure, but, that's not gonna sell out a venue most of the time.
a well run website, mailing list, and other things some promoters are able to do can bring in the people who are on the fence about it or don't know that the show is going on or when it is (or forgot about it)
(that's my understanding of it, I've been to lots of shows that i found out about from the marquee that day or an email from the venue)