Well if you want to reach lots of developers, you have to be where they are, and they're on Twitter and Discord—for better or worse. :shrug:
The biggest projects are typically on IRC, mailing lists, and online forums. Linux, ffmpeg, practically every Linux or BSD distro, Git, nginx, postgresql, systemd, neovim, Freedesktop, etc.
Practically none of the software I use on a daily basis has a significant Discord presence.
>neovim
Neovim is on twitter, and doesn't have a mailing list. They also offer matrix and IRC (presumably bridged). See https://neovim.io/community/
The best part about these, is that though we step off topic sometimes, generally everyone is really helpful and focused on the general subject matter. It's not endless gossip, in jokes and memes, these people actively find new faces, and then find places for them to fit in the community and things to take ownership of.
These smaller networks are a rare type of community, and I love it.
Humour aside, you would do it if you wanted to encourage movement of the closed silos. It's not likely that your irl friends and family will make the jump anytime soon, but getting dev communities to make the switch is a much more tractable problem.
But that has not been my experience with Matrix.