Do I sync my Facebook friends to my phone book. Absolutely.
Is that a problem? Not if you know how to filter it. Go to the Display Options of your contacts book and set display only those with phone numbers.
If any of my contacts using Facebook decides to change their phone numbers in Facebook, the new phone number is directly synced to my phone book. No problemo, he or she is instantly updated.
Its also cool to read their latest updates in the phone book.
Basically I like the Facebook syncs because it also fills my phone book contacts with their Facebook pictures. If they change their Facebook pics, the change is automatic on my phone too.
What does Facebook get out of it? Be aware, it means they can read your Google and phone contacts too, match them to their database and reveal their Facebook presence to you for your to friend.
And its not just Google they will peep into, but also your Twitter account if you let Twitter for Android sync your Twitter contacts to your phone book.
And Facebook will do this to your Yahoo Mail and Yahoo Messenger contacts if you let Yahoo Mail or Yahoo Messenger sync contacts to your phone too.
But the syncs are a two way street. It also means Twitter can read your Facebook contacts that have been sync'ed to your phone contacts, match to their Twitter profiles, and suggest their Twitter profile for you to follow. Ditto with your Google contacts.
And it appears to me so can others (Yahoo Messenger adding Facebook chats to my phone).
Don't you love the incestuous coupling of social networking?
Better yet, wait until you get Google+. The Google+ Android app will read all your phone contacts, including those Facebook and Twitter sync'ed to your phone. That lets you migrate your Facebook and Twitter friends to Google+ without using a web tool. Google+ will also suggest your Yahoo friends too if Yahoo Messenger is syncing your contacts to your phone.
Neat.
Seriously aside, it all sounds like privacy issues here but then again I already knew what to expect and what to give out.
The advantage of having FAcebook, Twitter, Yahoo, Google and etc,. sync contacts to your phone is that you can link each identity and join them into a single one.
John Smith with his Facebook ID, his Yahoo ID, his Google ID, his Twitter ID, his LinkedIn ID, etc,. in addition to his phone, email and address information.
This massive consolidation, for me is worth it. Someday, the social networking ID might be important than his phone number or email address.
What's neat is that I know what these my contact's Facebook and Twitter profiles even when they don't tell me what they are. It gives me a psychological advantage.
Note: by the way, Google disabled Facebook sync if you are a Nexus S user with an OS dated 2.3.3 and above. Google wasn't happy that Facebook is using the sync to suggest friends using the Google profiles without some reciprocation. They didn't disable Facebook sync on the Nexus One however, and with any other Android Gingerbread phone.