You cannot and you are right there. The only way to target individuals is by having specific lists of emails or ids that you can upload and create custom audiences (or via their notion of retargeting through pixel fires). Even then though you don't know WHO saw your ads. Its a walled garden and impossible to get per user data as you would in traditional retargeting campaigns. That is why the bidding system is impossible to game (despite some claims from some companies). You have no idea WHO is seeing your ads - only the DEMOGRAPHIC of the ad views/clicks and only if you segmented your campaigns in a way you can track that.