Brian is wondering if he can prevent third-party apps from seeing his contacts list on his phone. Leo says it's very common for apps like Snapchat, Twitter, Facebook, Instagram and more to suggest inviting friends from your contacts list. They have to ask permission to get to this list, though. These apps upload the contacts list to their servers so it can alert you that someone new has joined that app. Leo says that absolutely is a privacy breach. He could say "no" when it asks him for permission, but there's nothing he can do to prevent his friends from sharing his contact data to one of the apps they use.
Leo recommends that Brian not give out his actual phone number to his friends and family. He can get a Google Voice number, and use that instead. It will forward to his real number, but at any time he can change that forwarding. He can even create a Gmail account that's only for social. The problem is, he's giving a working number to his friends, so the trick at this point is to assume that his email and phone number are no longer private.