I am one of those who built and deployed an alternative to Facebook: nderground.net. I never saw nderground as a "Facebook killer" but as an alternative social network for people who were concerned about privacy.

nderground.net was live for about four years. It has a Facebook style "board" where users could post text and pictures for their group. It also supported a photo gallery that was targeted at being something like Instagram.

I had a feature that allowed people to import an invitation list from their GMail account (I planned to add additional invitation import features).

As noted in this post, nderground.net was never successful because it was too difficult to build a network on a new social platform. When people started on nderground.net they felt like they were all alone because there was no pre-existing group to interact with.

The premise behind nderground was privacy and security. I thought that this would be popular with people who worked in professions where they would be concered about stalkers (psychologists, judges, lawyers) or did not want to be public (people in the Intelligence community or who were in sexual minorities). As it turned out, privacy was not as strong a draw as I hoped.

In the end I have a few hundered people who signed up. In the end I had no users who used the site regularly. nderground was self-funded so I was paying the Amazon Web Services bills and after about four years I, sadly, shut the site down.

While not Facebook killers, there are other networks that seem to have been successful as Facebook alternatives. Discord is one of the primary social platforms that comes to mind.