What has been your experience with Facebook Groups, and what helped you build an active community?

A regular weekly thread can help get people talking. It gives members an easy way to join in without having to come up with a whole post themselves

I think facebook groups can be very helpful to build a community, but the biggest challenge is to keep active the people in community. In usual cases people joined the group but they remain inactive and usually they give less response in groups that's why groups dies soon. If you want to keep your community alive you have to find that topic which is the basic and daily need of people so that they can post daily likes share and also comments on others posts. This way any community lives a long.

Focus on sharing useful knowledge that encourages discussion; avoid posting an excessive amount of content.

Agreed. A post that contains factual information and sparks discussion is far more valuable than many superficial posts.

In my opinion, you should create discussion topics that align with the members' interests, organize weekly questions, and respond to comments regularly. Useful content will help the group grow naturally.