Dark Crystal[https://darkcrystal.pw] is perhaps my favorite implementation of this. It's built on top of Secure Scuttlebutt, and allows you to share private keys with trusted friends, so that you can later rebuild them when they are lost.
If you know of other insecurities, I'd be interested in hearing them.
Edit: apart from that, nanomonkey is right, if you post something publicly, it will be public. That's to be expected though. And assuming the use of libsodium is done correctly, there shouldn't be much of a problem with the primitives.
Now, the logic implemented on top of the protocol is a whole different story, sort of like most ethereum cautionary tales are not about ethereum being buggy, but people running buggy code on ethereum.