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 500 together, they're probably make up a sizable portion (if not the majority) of world GDP.