Get out there yourself and talk to people about the show in person. I understand we are in the digital age, however, that does not mean you can't inform people via verbal communication. You have a greater chance of getting someone to come to a show after you build even the slightest personal relationship with them.
Even if it is a “hey you should check out our band” and a smile afterward. You could make an impression and get that person to come to the concert and maybe bring friends.