They have the same customers and customers have a finite advertising budgets. When Facebook gets more Google data it benefits the "result" of the ads on facebook which in turn over time gets more $$ shifted there.
1. Who knows what data Google is giving back in return?
2. Neither want a monopoly. A duopoly is far more resistant to regulation.
3. Many business will advertise on multiple channels. So long as they edge out the others it's fine by them. They know they are the two premium outlets.
