The second most useful by far is Indeed, where we post an internship opportunity for participants interested in doing 10 sessions over 10 weeks. Other things that work pretty well are asking professors to send out emails to students at local universities, putting up ~300-500 fliers (mostly around universities and public transit), and posting on Nextdoor. We also just texted a lot of groupchats/posted on linkedin/ gave out fliers and the signup link to kind of everyone we talked to in cafes and similar. We take on some participants as ambassadors as well, and pay them to refer their friends.

We tried google/facebook/instagram ads, and we tried paying for some video placements. Basically none of the explicit advertisement worked at all and it wasn't worth the money. Though for what it's worth, none of us are experts in advertising, so we might have been going about it wrong -- we didn't put loads of effort into iterating once we realized it wasn't working.