I'll get maybe one or two sales when I post an announcement about a new thing. There's no knowing if people visit the page at other times and follow links. Used just for big announcements, it doesn't take long to update the page, so it's not a big time concern. I'm dubious about the value of spending a lot of time on it, precisely because the greater the success, the less likely it is that followers see announcements. Facebook actively punishes people for success in the hopes it'll force them to pay for advertising.
Other sites never replace having your own author website. As for other social media, I guess it depends on your success. If you're someone who threw a book on Amazon and suddenly you were a bestseller, you probably don't need it. If you're a mere mortal like most of us, Twitter is useful. I haven't had many sales, but the majority have come from Twitter, in one way or another.
