I see a lot of alarmist notes going around about the Facebook Contacts List as if it is a big invasion of privacy. It's not. It is as private or un-private as you want it to be.
If you want to see a list of your friends phone numbers (of the friends who have posted phone numbers for their friends to see on Facebook), go to edit your Friends List, and click on Contacts. Notice, you will only see your Facebook friends phone numbers (and not the phone numbers of everyone else on Facebook).
If you want only your friends to see your phone number and/or your mobile/cell phone number, you can post these on your Facebook Page (with your info). If you want anyone, including people you don't know, on Facebook to see your information, then you can adjust this in your Facebook Privacy settings. If you don't want anyone to see your phone numbers at all, don't post them in Facebook.
There is no scary privacy issue here. Whether your phone number appears or not, and who can see it, is up to you.
If you have an iPhone (this might work on other smartphones too), and you find that suddenly a lot of non-facebook people's numbers are on your contact list (your whole contact list from your phone), then you would have allowed a Facebook application on your phone to do this. There is not a lot of harm in this happening as this list is private for you only. However, if you have email addresses in your contacts of people who are not on Facebook, these people will receive automatic invitations from you to join Facebook every couple of weeks. You can disable this reminder on Facebook here.
If you don't want all your contacts from your phone to be in your Facebook on-line too, you can remove them by going here and following the instructions.
You can keep your number and your contacts list private in the Facebook Account Privacy settings on your Facebook page.
Some people have their privacy set so nothing is private. Then anyone can see their private info. These people are either unaware, want everyone to see their private info, or they don't care.
For more details on this, go to this link.