best community platform for member engagement
Facebook groups is easily the best community building platform in existence IF your primary goal is member engagement. Everyone is already on FB daily, so just one click or notification away from your group. I've seen countless community builders try to migrate a community off of FB onto a standalone platform -- and engagement drops off a cliff.
The downside is lack of control, branding, content ownership, membership dues collection, and search. There's no great way to organize a knowledge base of content.