cautious automatic updating of my address book from my Facebook, LinkedIn, Xing, … contacts

If I really have time enough one day (i.e. also the money to afford that time), I would love to use those APIs to collect phone numbers etc. from there and update my address book with these details. I would really like to have that. But I already see problems like this:

• some "friends" never update their details, so the details I already have are always more uptodate, then the ones they publish in the social media profiles,
  so maybe I am able to integrate those details with custom labels (to signal, where they stem from), but I would not be able to automatically update my smartphone address book from that
• …