I am sceptical of the idea. If every post had a specific license, it would be a minefield to federate or host, because every post could potentially forbid sharing it, or have other stipulations.
Most posts and comments are not copyrightable anyway. A few sentences are not enough to count as a “creative work”. It would need to be your entire posting history, and even then its dubious if that counts as one work.
I propose instead that we do it like wikipedia and others, and that the server as a whole has a license.