wordpress plus buddypress - both free / open source. I've been wanting to find someone that could package those together with a few other plugins and settings to shore them up (added security and anti-spam settings) and make it a one click deploy for people via the popular services like DO, vultr, etc.

I think it would be easy to get one person in a family to pay for hosting while another person keeps an eye on updates and backups. Same with groups of friends, even if most are broke / not willing to pay, if just one would pay it would be trivial to host a few hundred friends on one install and a few hundred family members on another.

Buddypress is mature enough with enough plugins to do most of what fbook and similar offer all for free. Only thing I don't really know is what kind of mobile notifications options are out there and people would expect or need, as I don't use fbk the same way others do.

But groups, pics, vids, messaging, profiles, privacy settings, these things are fairly easy with BP really.