The nature of advertising will become more "native" and will make it harder to separate ads from the content. We are already on our way.

Real businesses will be mostly unaffected. People will still need to buy airline tickets, insurance, widgets, etc online.

If facebook collapses nothing of value will have been lost. If google isn't able to stay afloat without ads despite billions in R&D funding then they were never meant to be. Life would go on.