This can be done from the Facebook application. The sync feature can be found by pressing the Option button > Settings and swiping to the bottom of the page inside the Facebook application. Under Other settings section you can find the Sync Contacts feature that you can use. Tap on it and choose Sync all. After you did this, your Facebook friends will automatically be synced with your contacts.
If you can't complete the process, go to the menu of your device > Settings > Applications > Manage Applications > All tab, search for Facebook application, tap on it and tap on Clear Data and Clear Cache. After you did this, try again to sync contacts.