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.…
What would that payment be for from Facebook? 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.)
It really does. Want to search for a local house cleaner or children's entertainer, include Facebook pages with web results. Want to search for things to do in a particular town, include Facebook events with web results about museums and such like.
This is just scratching the surface too. There are lots of possibilities here so I'd be very surprised if Facebook haven't explored this idea.