If you see your competitors (or people will similar products or services) using a particular medium it's usually a good bet to at least try. But even if you try you have to give it a test of quite a bit of time (and that depends on the product and amount of profit as well and other factors).
A good example of this is "back in the day" yellow page advertising. If you look at what the YP called "a developed" heading (like Plumbers/Printers/Lawyers) you will see many ads year after year that vary in size including many full page ads. So that is a sign that's a good bet for advertising if that's your business. Especially when you find out the high cost. And I built an entire business based upon starting with a small ad in a developed heading in the yellow pages. Ad went in. Calls came in. (Expensive but worth it. Picked up a major account and did it for maybe 10 years until I sold the company). To me it made sense that if year after year small businesses were spending big dollars to be in the YP that was proof that it worked (and to mention again it did work extremely well for me). Of course when your competitors who don't advertise ask you you downplay totally the value and bitch and moan to throw them off the scent.
I would suspect FB works the same way. (In fact that was my experience using fb for a particular product but I can't say I gave it much of a chance though but take that as another data point).
I also had a similar experience with "card decks" those mailers that go out to businesses offering products and services. Worked very well. Sent out a card in multiple decks for a product that others offered and leads came back just like that.