There are several OSS social networks. I think part of the problem is that to "replace Facebook" it would have to be as addictive or worse then Facebook, defeating the purpose of replacing it.

In other words: Why hasn't science found a way to "replace" heroin?

Good question, maybe one day we’ll find a way to increase dopamine production and not have brain chemistry react to it the same way it does now. Which is to say, elevate mood without dependency, it doesn’t even have to be a chemical.

The problem is the users. Facebook is popular because everyone is on facebook. Tons of new social media sites pop up daily, but not many people use them, so the herd sticks to facebook because that's where all their friends are. The best way to combat this would be through interoperability where a person could use facebook, but through another site. Not needing to be on facebook to use facebook is /directly/ in opposition to the companies business model though, so they'll constantly find ways to combat interoperability. Even sue against it.

I think the open source aspect of this is irrelevant. I think the question you're asking is why hasn't anyone replaced Facebook with a better alternative AND why hasn't it caught on.

See wt.social for an alternative. Who knows if it'll become popular but I bet it won't be because it's OSS or not... simply because your family and friends don't know what that is or don't care.

my take is because it doesn't care. the kind of problem that facebook solves, socializing for social butterflies is not appealing to developers, who tend to be introverted.

Not worth the effort?