The analogy to predatory pricing doesn't work because in the brick and mortar world, you drive out your competitors so that you may raise your prices. So the price is said to be artificially lowered.

In the software world, free software products stay free even after they have become the dominant platform. The price is not artificially lowered to zero; at scale the cost of operating a free service is lower than the advertising revenue generated by it. This is actually the "natural" price for the product.