By providing a product/service that helps them streamline their workflow or sales process. Something that's recurring like a SAAS is my go to. Something that takes away a pain point is the key.
There is a youtuber by the name of Jason Waldrop who does specialize in Real Estate lead generation. That would be a great source of ideas to apply to your FB community. https://www.youtube.com/c/JasonWardrop/videos