Davis [FluxBB] uh no. Flarum is to replace it.
I was wondering about this too, because I know the Textpattern CMS community relies on FluxBB heavily. Their entire history is written and archived in their boards, and highly valued. They are aware of Flarum, but reluctant at this point to migrate to it because of the lack of progress and clear roadmap. (Yes, yes... I'm told the gears are turning.)
It occurs to me that a boon for Flarum is to get existing OS communities using FluxBB to migrate to it sooner than later (which means getting a stable out the door sooner than later), because those communities, which have skilled devs, generally take loving interest in the communication tool they use. As a result, Flarum might see more core code contributions, more extensions development, and more feedback in general about Flarum's direction (smart feedback about sensible code, not feature requests). I'm not saying there aren't smart devs here. I'm saying there could be more smart devs here through more devs in general by getting those key FluxBB communities.
What can Flarum do to get their attention and entice them over from FluxBB to Flarum? From a "marketing" angle, Flarum should be thinking about that. Targeting copy in the website (i.e. speaking to those communities directly as one such audience). Better documentation (both dev and user docs). More regular blog posts. Better use of social media (Twitter, Facebook, whatever)... Things that other members of Flarum's "team" can do besides devs themselves. If you just rely on these boards and a static website, you won't get far with reaching those audiences quickly. But you all need to communicate to ensure the right message is getting out and the documentation is current and accurate, as examples of team harmony. Of course, on the dev side of things, they need to get that migrator developed and working sharp.
I've seen a few people from the Textpattern community interacting in older discussions here. One being Textpattern's lead dev (he didn't identify himself as such in the conversation), for example, who was having an exchange with Franz about code. Another, a long-time community member and forum moderator, was asking about Textile support (Textpattern's forum has used Textile for many years. The CMS too, though in the upcoming 4.6 release an author will be able to choose Textile or Markdown on a per article basis). And another who runs a Textpattern sites/themes project, who temporarily tested Flarum for that community. In all cases, the conversations weren't going well, but the Textpattern community will have to make a decision eventually.
Textpattern aside, there are surely other OS communities using FluxBB. Court them before they choose a competitor without know you exist or care to have them onboard.