I think it's the narrowcasting that is a danger to democracy. When you have to blast your message across popular broadcast media your message has to appeal to a wide cross-section of the public. The ability to secretly target individuals you can craft messages that would be repulsive to a larger audience. That's why Facebook's political ad transparency is an important first step against this new attack on democracy.

Ouch, that other benefit of narrowcasting (the relative secrecy of it) didn't really click with me until now.

This is so true. Also compounding the problem is how we're so isolated into our own interests and beliefs, so even if you share your personalized propaganda it's likely to be with other like-minded people. It's like we're all living in our own dream worlds.

What is this supposed to mean? Can you be more clear about the point you're trying to make?

Alice, Bob, and Carol all hate each other. A politician of yesteryear had to forge some compromise between them to get the support of any two. Today, you can tell Alice that you'll kill Bob and Carol, Bob that you'll kill Alice and Carol, and Carol that you'll kill Alice and Bob, and get all three of their votes.