Startups used to be obsessed with billboards. It was the first thing I noticed when I moved to San Francisco: venture-backed companies including Eaze, Airbnb, and notoriously, Brex, would post large billboard advertisements all over the city to grab attention and eyeballs. When I dug into it more, I learned this type of old school, outdoor advertising was a response to the increasingly crowded online channels, such as Facebook and Instagram advertisements. Well, folks, years later, we have a new response to crowded marketing channels: Ditch the billboards and just…