They compete in the same market (digital ads) but provide different utilities for the consumer, hence aren't competitors when it comes to user attention. Google provides a search engine, GSuite stack and other free products, YouTube, etc. Facebook provides a social network and networks to connect with their families. There isn't a meaningful competitor for either in either space. And most companies simply retain two marketing budgets - one for Facebook and one for Google.

Exactly, it's like with the Cable providers "competition". They theoretically "compete" because they sell the same, but in reality, geographical availability is set in a way that they are focused on servicing different demographies

They compete with each other in the same way the McDonalds and Burger King compete for the cows that are made into hamburgers ....

Google's and Facebook's customers are the advertisers, and they compete for those. But users are the product from which the burgers are made.