YouTube pays because they're displaying copyrighted works. I don't think even the worst case— displaying opengraph tags that sites have to opt into, meets the bar for reproduction of a copyrighted work. I mean we're talking about linking here.
It's not that the marginal cost of journalism is a pittance, but that the marginal value to a social network of links to a few news sites is actually zero. I would take a bet that if social networks just up and banned all external links period it would be at worst revenue neutral.