I honestly don't remember when last i heard that platforms like Facebook, Instagram and even Google suffer from a DDos attack, it's been a very long time I heard of such and I guess that must have been when the company was still growing, even the google outage you are talking about is also something I know nothing and have heard nothing about, except there is the possibility of the platform experiencing an outage in country or continent while it's available in another.
I don't think a small or medium DDoS attack would be able to slow down any of those mentioned companies' servers, as they are now giant companies. I also do not remember the last time I heard their service was down. Even if some of their service goes down, that is not because of the DDoS attack but due to technical issues or server upgrades.
As for companies that do not have their own servers, they usually use 3rd party hosting such as Hostinger or other international hosting companies. Also, they use DDoS protection in case they get attacked.