Donate some of its engineers' time to fix and revitalize Diaspora -- https://en.wikipedia.org/wiki/Diaspora_%28social_network%29 -- or one of its peers or something along its original vision.
It could achieve its goal of disrupting Facebook, give users their privacy back, and by releasing the source code, could role back its role in maintaining the code or policing the community.
It could declare victory and move on, leaving its users more satisfied than they are now.