The other advantage that I didn't fully realize until I started running the ads is how the activity within the Facebook Event serves as a reminder for people to show up - we see tons of ads for events every day that we want to go to, but very few of them ever make it onto our calendar the way that Facebook Events do with one click.
My argument isn't that Facebook Ads are the be-all end-all marketing channel - just wanted to share my experience with them. YC folks - feel free to ping me if you want help spinning up some Facebook Ads - always down to help folks out.