I do appreciate the method, I just find hard to believe that it can be put in practice in realities like mine.
To find out where to start, ask yourself where your children are more likely to get caught alone and reported to CPS.
It will also require you go to clubs (sport club, book club), events (garage sell, protests, etc) or even to put them in motion.
Eventually, some thing that works very well is talking to people waiting with you. In queues, waiting rooms, at traffic lights or even traffic jams. It's awkward/annoying as hell when you do it wrong, but after a big of practice it's really a great source of networking.
You mention Facebook - I think FB could be leveraged here as well.. perhaps a neighborhood group.
But you can mix both approaches;
You can do one-on-one to create / maintain the relationship, and utilize social networks to organize the community.
But if you want your local baker to go out of his way to get your child out of trouble, you need to look him in the eyes when you ask for it. And you need to do that for each person you recruit. There is no way around it.
Actively socialize a lot with people in your neighborhood. Including the old lady next door, local cops, teachers and shop owners. And once you know 50 persons around by name and they have your phone number, tell them one by one that you intend to let your child going out more and more out by him/herself to learn autonomy. Ask them to not worry, but to keep an eye open: they will feel important, think you are a wonderful parent and your kids will have 50 people to help them in case of a problem.
In my experience, if you befriend on average one person a week, you get there after one year.
Manichean but works wonder. Learned the trick while working in Africa.
It actually works for many other things: finding a job, getting help to do stuff, getting laid... You usually make real friends on the way, not a bad side effect.
Even in those facebook times, one-to-one human relationship are still ruling everything we do, and living in a community is one of the most rewarding skill to sharpen.
But yeah, we are not in a movie, it's a lot of work. The maintenance part is really time and energy consuming, as not everybody is interesting and worth it.