Common sense suggests that you should not be posting on Facebook what you do not want the world to know.
While Facebook denied that it is directly selling phone numbers to advertisers, and is using the phone numbers only for security reasons, it only said half the truth in a move that is best described as don't ask, don't tell. An entry posted to the Facebook developer pages provides some information that phone numbers are, in fact, used for custom audience targeting. According to the post, phone numbers and email addressed can be leveraged, provided they already generated this data, either from within Facebook or through other means.
Advertisers will have to hash the data to target specific audiences and avoid scatter shooting to make their advertising more effective. Facebook claims that it does sync phone numbers, but it does report a number of hits, albeit not a list of user names.
If you guard your privacy on Facebook with reasonable measures anyway, you may already have questioned the reasons for providing a phone number to the social network already. However, for those who do not and may not be overly cautious, Facebook can certainly be accused of not be entirely open about the phone number issue.