In my humble opinion, if a venue wants people to come out and see a band they need to do the work of promoting live music at their venue whether that be a restaurant bar or club. Of course every band is going to do their best to use Facebook and other social media as well as straight up invites to get people up to the shows, but the thing is while most bands develop a following, we're not the "big time" for the most part, and so it's up to a venue to be in charge of promoting their shows. Like who they think you guys are freaking Ed Sheeran or something?
This can be frustrating for sure. There are two places we play that hire bands every Friday and Saturday night. They pay OK, but not great. When you look at their web sites, the band calendars are a year old. Their Facebook pages haven't had activity in weeks. One place had, on the day we played, a sign outside that listed as the night's entertainment the band that played there a month ago. We never get a lick of promotion from either of these places. We bring in as many people as we can, but damn, they need to at least put in a few minutes per week to help themselves.
You might get a few of the 'interested' crowd as a bonus. (Can't say with certainty how many bodies the old school postering method might be bringing in, if any, in this online age... but it's fun, and they make great wall art for the rehearsal space