Only out of convenience. There are many alternatives, and search engines will find them. And (although it won't happen) if Facebook were ordered to shut down, hundreds of entrepreneurs would offer alternatives for "small business web presence"
Not everyone has SEO-fu, and hiring a contractor is an additional cost for small businesses. At least on Facebook one normally finds what one is looking for.
Angie's List started out this way but I think it has been corrupted since the merger with HomeAdvisor. I could be wrong about that.
Establishing trust would be the tricky part. People are so used to 80% of the internet being scams and clickbait. But that certainly goes for Facebook pages as well.