Just remember the client itself is proprietary (as their business model will be client features), but the bridges are open source.
So you can self-host a facebook bridge and matrix server, but you’ll need to connect to it from a matrix client. You can’t do it from the beeper applications.
The Beeper application is a fork of element, but you should be able to use any matrix client. In the past I’ve self-hosted the facebook bridge and successfully used it with Element so don’t let it put you off 🙂