Facebook failed to pay the big money to Apple that they should have been paying for years. If Zuck had been handing Apple $5,000,000,000 per year as google does, then Apple would never have kneecapped Facebook. Larry and Sergei know how the protection racket works. You pay your dues to the local mob, you get to do business in their street corners. What do google pay Apple $5b for? Ummm… to be in the search of Safari.…
Opinions aside, Google are paying for a thing, not just donating money. What would Facebook pay for? I'm not sure that them being a search engine makes sense, I don't think Apple would give up the position of iMessage in favour of Messenger.
(Disclaimer, I work at Google, but don't have any inside info or opinion on the Apple/Google relationship.)