Unfortunately, businesses have to go to where the customers are. Many customers now are 100% Facebook-era, particularly true of the young and those older folk late to computing, and it might not occur to them to check
https://www.widgetsRus.com before searching for Widgets R Us on Facebook.
Updating a 'personal' site is also a PITA IF you don't know what you are doing or don't have a streamlined process, e.g. for me I'd just amend the .htm page to say I'm shut from 2pm and FTP it up, 2 minutes, but someone non-savvy will always prefer to post a new message on FB saying 'hey, shutting at 2pm today', takes even less time and less cognitive load.
I'm not saying this is a good thing, by the way. But it's understandable.
EDIT: Not to mention, updating a site on the move is difficult. Pull latest version of site (Github), amend .htm (some notepad app?), then an FTP client on the phone, log in, find, push the file, close, check. Versus opening the FB app and doing some typing.