I deleted my Whatsapp account from my phone and my wife's as soon as this news broke for this precise reason. I don't want Facebook having my telephone number, IMEI, router information, etc.
But you make the very good point that all of my 2 dozen or so Whatsapp contacts that have my phone number will be giving it to Facebook anyway. As we are all well aware, Facebook's backend is VERY good at identifying who you are through its analysis of social networks [viz. the 'People You May Know' feature], so they will likely be able to fill in phone numbers for basically all of their users using Whatsapp's database, even if those users do not actually have a Whatsapp account.
When I lived in Egypt some years ago, before the fall of Mubarak, I used to hang out with quite a few anti-regime activist types. They would organize pathetic little demonstrations, frequently via Facebook. And every once in a while, if they were organizing something that the regime really didn't want them to do (demonstrating at the Interior Ministry or something), the government would come in and efficiently round up everyone who had checked in or whatever via Facebook, before the demo got started. It was pretty clear that they had penetrated the online social networks pretty thoroughly, either with or without help from Menlo Park. I've had a very healthy skepticism of Facebook ever since.
It's funny how when we were kids (and for generations before), the bugbear was the all-encompassing government surveillance state. And it has arrived, but it crept in through the ethernet port, with our own little voluntary checkmarks next to the User Agreement. And it came through private companies like Facebook and Google.
The ship has sailed.