Booking.com has in the meantime almost 40 FFP partnerships – and it is definitely worth comparing them since very different accrual rates apply across them. The latest addition, Etihad Guest, applies thereby one of the most rewarding ones, between 3 and 6 miles – depending on the property – per USD spent. Many other partners apply rather 1 or 2 miles per EUR/USD. However, you should not be in a hurry to use these newly earned Etihad miles: According to their website, they are credited only 75 days after check-out. Probably the accrual information is transmitted by falcons.