Don't put your number on your profile then
"I looked at my contact list and I have dozens of private numbers for people I don't know" that's because those people put their number on their profile, this is not the sync issue they are talking about.
I have the option to put my number's on my profile or not, and what I do have is shared with friends only minus a couple of groups I have made that I don't want to see those details.
Separately the facebook app on my iphone has sync'd my phone book with my contact, for people I already had the number for and for people that on their profile have added their phone number.
Also if your worried about your number getting out to other storage places, sorry but you don't think people that take your number don't stick onto the cloud somewhere? All my contacts are on google already, address, number's, dob, IM, twitter, job title, place of work.... welcome to the 21st century.....