Earlier quoted context omitted.
It's not quite half a percent by most accounts of GWP, but it's around there.
0.5% of global GDP would be more reasonable, although a fraction is an odd way of writing it. It also doesn't tell us much other than Google and Facebook are large companies with significant earnings. You can probably provide a similar percentage for VW/Toyota, Boeing/Airbus, Walmart/Amazon, or any other pair of large companies that are the leaders in their respective markets. Indeed, if you lumped the entire Fortune…
Seriously though, Wal-Mart has $500 billion revenues but it's not growing at the rate of these companies. By lots of metrics these companies are increasingly globally significant in a way that most of the examples you quote aren't.