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"
So millions of small business owners would have to pay for something they get now for free
Small businesses have complained about being buried in search results for long years. In some industries like restaurants, scammers create a higher-ranking website that looks like the real thing, in order to skim revenue.
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.