I agree with the comment re: defraying costs, but Open source software has been able to find a way to survive and THRIVE with a not-for-profit model, so there must be a way to do it.

Decentralized and for-profit is an interesting idea, but my main concern is that a for-profit model gives the organization a reason to mine the data and monetize it, which could be done even if the hosting isn't centralized.

The reason I like the not-for-profit model is because it tends to (partially) preclude some of the shady privacy intrusion that the for-profit model could push a business towards.

Open source does not thrive on the fuel of non-profit alone. Red Hat and Sun are for-profit, but they're major contributors to open-source projects.

Open source is successful with not-for-profit models because people like hacking on open source. I don't think that's necessarily impossible for a social network, because people like to waste a lot of their lives on social networks, but are open-source hackers going to be interested in working on a project they aren't interested in using? I'm assuming that the primary appeal of social networks has long moved past the techie demographic.