A myriad of small businesses also use Facebook as their public facing website.
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.