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I use an app called Smart Sync or SmartSync.

But I think the Facebook app added this functionality recently, check it out.
 
Yep FB added this feature. Just go to the the Friends screen [via iPhone app] and tap on Sync in the upper-right corner. I believe they update whenever a profile pic is changed as well.
 
This looks like a cool feature, but I don't think it'd be right to violate the privacy of my contacts by sharing their phone numbers with facebook. What if they don't want their information shared? Is there a way to only sync a select few?
 
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