We did replace it, almost 10 years ago. It was called Diaspora[1], no one cared about privacy then. No one really cares about privacy now, at least not enough to do anything about it. 1. https://en.m.wikipedia.org/wiki/Diaspora_(social_network)
Part of the reason (although not entire) is why many users like Snapchat because it doesn't keep a history of your messages. Snapchat doesn't use targeted ads like FB does. That's why everyone gets Kylie Jenner ads instead of personalized marketing