I feel that Discord is a better way to create a group than Facebook.
Facebook can be a great way to promote your forum and engage with your community on a different platform. Which can help drive traffic to your forum and increase engagement among your community members. Discord can be a useful tool for building a sense of community and fostering a sense of belonging among your members.
However, it's important to keep in mind that a Facebook group or Discord is not a substitute for a forum, as a forum allows for more structured and organized conversations around a specific topic, whereas a Facebook group or Discord is more of a general space for socializing and sharing content.
That being said, I think Facebook started declining over a year ago, so Discord is a better choice.