That already exists. It's called social media and people promote real life events there.

Well, most platforms operate primarily as an algorithmic scroll based feed, and the actual utilities (like event planning, or marketplace) are related to second class citizens – used mostly as a hook to get you to stay logged in.

I've had a lot of success lately relying exclusively on Partiful as my one social app. I know it's nearly an inevitability though before they will need to monetize and introduce some way to ruin the elegance.

(My proposal for the modern successor to Zawinski's law: Every social media platform attempts to expand until it has a scroll-based algorithmic content feed).

Distribution, getting people to know that the event exists is the hardest part. Scroll based algorithmic feeds are good at helping you with this.