This isn't a problem you face alone ... and I think it's telling that Google has had to put more ads on each page to grow revenue. In their case, the ads are targeted to the user AND often are related to a product search. I think FB has a bigger problem because people didn't go there to search for a product but rather to engage their network.
So, despite being certain that our target audience is on FB, we have concluded that we're better off advertising in a place where the intent to purchase a hotel room/travel is clearly present (ie google search).
It's a specific situation, certainly not true for all advertisers. I'm sure that FB advertising for other products might work quite well. It's just not for us.
That person has already replied with more info on his case, but you should also note that for some people $20k is big enough to never risk that mistake again, while for others it really isn't. I spend around that much each month on Facebook and that's a "well we'll put a little into FB" amount in the scheme of things. Sure, I've had other clients for whom a badly spent $1k was a problem, too. My point is to not jump to assumptions that an adspend is big because it sounds big to you.
(He'll, just this morning I sent a mail to a video website saying that a $15k spend delivered shit results and that we should try again.)