Facebook is going to be your predominant answer. Venues and promoters are the best bet for specific cities. Band pages and labels obviously help too but I personally follow very few on FB and get by fine. Those types of pages are far less localized and just clutter things up for me I find. You don't even have to write a bot, Facebook already does all that shit for you.

Event pages also recommend similar events, which are typically other events put on by the promoter or at the venue. Sometimes promoters will post about other shows they have coming up in event pages. Just don't be afraid to turn off notifications for specific events because god damn some promoters post like once a day for each event and it's maddening.