OKSocial is funded by donations. We considered just making a paid service but ad-free, privacy-focused, social media access shouldn't be restricted only to those who can afford to pay for it so we're trying out donation funding first.
As a small operation, our costs per user will obviously be higher than those of the larger commercial social networks. We also want to be able to contribute to the continuing development of the diaspora* and Pleroma codebases and help fund the development of a diaspora* mobile app.
We chose our recommended donation of $2.50 per month because (minus processing fees) that's about what facebook makes off of each user by mining their personal data to optimise their advertising. We don't mine your data and we don't advertise at you so that's clearly what the market thinks privacy on an ad-free social network should be worth, right?
We're also well aware that even that amount is difficult for many people to spare, so please donate if you can, but don't donate if it would be a hardship for you.