What incentives are provided by Upwork to contractors to increase activity on the platform?

Absolutely. As I mentioned, there’s a step function decline in the take-rates to help reduce dis-intermediation. The take-rate model is aligned to the delivery of value . It's highest when you have the initial match, because the supplier just found work, there's huge value being delivered. Then, as you continue working together, because the supplier and the provider are building trust in each other, the platform take rate goes down. As the work-volume increases, you keep decreasing the take rate. 

At the same time, the platforms put in these other tools, like the automated invoicing, automated payments, the 1099 filing, a single platform for all your tax work etc. that kicks in. But bottom line, yes, as the amount of payments for ongoing work increases the take rates would typically go down.