Just an idea: since the utility of this is so dependent on the number/quality of the people using it in a certain location, you might want to start with hyper-f

Just an idea: since the utility of this is so dependent on the number/quality of the people using it in a certain location, you might want to start with hyper-focused groups (e.g. conference-goers, college cohorts, permaculturalists, church members) and then grow from there. Otherwise, if random person A visits the site and sees posts from other random people with whom s/he feels no connection, they probably aren't coming back. Good luck!