You say you have a feeling. But can’t we speculate a little on the way in which Facebook’s “priority inbox” will be better than Google’s? Facebook has information, sure. But more importantly, they have your list of friends. And the list of their friends.
Let’s say the priority inbox allows through emails from your friends and friends of friends — two degrees out. That might encompass 100,000 people for a well-connected Facebook user, a universe of most of the people you might want to hear from, with most spammers and trolls on the outside.
If you got an unwelcome email, you could presumably trace the connection and warn or delete the friend who kept such bad company. In fact, it would work much like people’s real lives. That’s a recipe for success — one hard for Google to replicate.