It's not quite half a percent by most accounts of GWP, but it's around there.
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 500 together, they're probably make up a sizable portion (if not the majority) of world GDP.
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.