Maybe for marketing? Because if they make this fight to happen their businesses will grow more bigger. Then also maybe they like to be written on history that Billionaires CEO fight in cage, this will blow up the whole world and can catch a overwhelming attention to people globally. This is a win win situation for both if they make this exhibition fight to happen. But same as other I doubt they are serious about it since maybe they are just having fun with the noise they created.
Absolutely not, nobody will use facebook/instagram/whatsapp more than they are using right now just because Mark fought Elon, even if he wins or ends up losing, the usage will not go up. And nobody will buy tesla car just because Elon fights against Mark neither, the ones who wants to buy one, will buy one, and that's it.
Business has nothing to do with this, they are child at heart people who made their money at a young age and they never grew up, they never needed to grow up to be fair and that is why they are fighting like children. Plus, these people are super wealthy, literally as wealthy as it gets and never will need money again and surrounded by people who say yes to them, so when someone challenges them, it becomes an ego war for them in the end.