It's actually custom software (based on Django/Angular/Postgres/Yocto Linux) designed from the ground up with an emphasis on privacy and security.

It will be open source, but interoperability with other systems is not currently contemplated as that would negate all the security benefits.

Not a word about the software they intend to use? I'd hope it is a nicely repackaged version of pump.io or Diaspora, something that might have a chance of being compatible with others.