This. Before, if you wanted to advertise a product, you'd say "Alright, so we've figured out that women 25-39 are likely to buy this product. Some of them watch XXX show, so let's spend a crapton of money to try to advertise to them in these limited ways, and maybe some of them will see it. Unless, of course, they fast-forward DVR through the commercials. Or choose to watch this different show. Or mute the TV to talk to their husbands during the commercial breaks. Or..."
Now, they can say, "I'm going to advertise to women in this narrow age bracket, who have these specific interests, and they're going to see my shit EVERY DAY for potentially HOURS when they're viewing Facebook. And if I can somehow convince them to click the "Like" button on my page, I'll show up directly in their newsfeed between their sister announcing she's pregnant and their friend's drunk ranting about her boss."
You seriously can't even put a price tag on that.