I hear the same is true about Facebook.

Yet both cannot be logically true, and Facebook doesn't make their own browser (though their app does use an embedded version of Chrome).

Imagine seeing "the internet" like a gigantic bubble bath. There's some very large bubbles, like Google, Facebook, and Microsoft till some extend. Many can overlap, touch each other. Some small ones get absorbed in larger ones. Some are more omnipresent than others. But right now, there's still other, smaller ones, and new internet-related startups exist every day (little bubbles). But not all of the small or medium bubbles become large ones.