Missing friends from any of your social network friends/connections list in Sync.ME, may have to do with your friend's privacy settings on those social networks.
For instance: If your friend declined to give access to third party apps on Facebook then he/she will not appear in your Facebook friends list in Sync.ME. The same is for LinkedIn and Google+: In Sync.ME the Google+ list is limited to the circles that the user made visible to third party apps. More info about Google+ settings for third party apps: https://support.google.com/plus/answer/2485911?hl=en
In addition, Facebook's new API restricts the amount of Facebook friends the app can show