I ‘inherited’ the co-ordinator role of a social singles group for people in their 30s & 40s in the NY area (no secret here, I have mentioned this on this board before). We run several social events per month, no cost to attendees beyond the meal cost/greens fees/billiards fees/etc - you get the idea.
It’s former modus operandi to get new members was to advertise selected events in the free singles columns in the local daily & weekly papers - but this has dropped off considerably over the past few years as newspaper readership (especially in the under 50 category) has declined. I am posting event notices on various on-line event calendars (such as News-12, for example), but I haven’t noticed any interest coming from those areas - and Craigs List seems to be a huge black-hole -I’ve posted a number of events over the years that should appeal to key demographics, such as Billiards, Bowling, Horseback-riding etc, with no response (I used a special e-mail for reponse tracking purposes, being such a clever boi… 
).
So I am asking for suggestions for (well, ‘public service’ no-cost) methods of advertising, preferably on-line (we’ve tried posting flyers in diners & supermarkets over the years, with a poor response rate - and bars around here tend to be hostile to most singles groups posting fliers for events not in their bar).
Success stories and strategies are most welcome.
Did you try starting a Facebook group? If any of your current members are on Facebook, ask them to join the group. Then their friends and friends-of-friends might join up. Make sure you actually post messages to the group on a regular basis, or people will forget about you.