Abdul recently told me about a Facebook group created to tackle a spate of break-ins in the local area. Once the issue was resolved, the activity in the group declined. Then someone suggested to use it for other issues, now it's thriving again.
You can identify the single biggest issue that affects your town (or street), industry, your field of expertise, hobby, or your company and create a mailing list/Facebook group about it.
Invite a few people to join, initiate discussions, ask people to suggest their ideas about how to resolve the issue. Build a consensus to resolve the issue. Implement a plan to resolve the problem. Update people with tasks for people to undertake and milestones to achieve.
Once resolved, you can use the connections and energy you've established between members to turn a single-issue community into something far broader, and far better.
The hardest part is always building those connections between people. Once you've built them, don't abandon them. One of the simplest ways to build the connections is to create a community of action to tackle the single biggest issue.