Our intention is that Freenet will only consume surplus resources, but we plan to build a reputation system that could have a notion of "karma" that is earned by providing resources to the network. This karma could be used to gate access to resources, for example like a VIP chat room on River.

So there are a lot of possibilities but for now users are motivated by a desire to see the network succeed, and that seems to be a sufficient motivator at our current scale.