Cold calling (or, rather, cold-leafletting) is not effective. Only two people on my block signed up, and those are people with whom I have talked face to face! I’m guessing that Nextdoor (and other social media groupings?) are, in this context, an extension of already-existing, real-world connections. I’ll have to actually meet the folks!
Edit (the next day): On the other hand, I have already received 4 RSVPs for The Disaster Games from people in “nearby neighborhoods” that I would never have been able to reach otherwise. I think the value of Nextdoor show up in the pre-existing, local membership, which affords instantaneous wide reach.