We won't be recommending production use until the beta period is over. That said, nothing stops you from using it in production now. Just be aware that the extensions available, and those that you create, may break with future beta upgrades. That goes for your data as well, there may be breaking changes that require manual upgrades, including mucking around with databases (or the simpler route, just to dump all user data and start fresh).
Mostly, though, we want to make sure that no one's sites are compromised because of bugs or holes in our software before the software is ready. If you're willing to take that risk, by all means, venture forward. There are many who do, and run sites right now on Flarum. Just be aware of the risks and that Flarum holds no guarantee of any breaking changes should you choose to upgrade, or security holes should you choose to stay on the version we have now.