A few things to be aware of now that this is live:
Right now, the Share to Facebook option when posting in a forum is on by default, so please make sure you understand this. This feature remembers your last action, so if you deselect the Share box, it will stay that way until it's clicked again.
The other issue: the Share feature appears in private forums, but anyone who tries to click into the thread from Facebook will not be able to do so. The only content that would be displayed on Facebook is the first sentence of the post -- anyone who tries to click through will either be prompted to log in to Mothering, and if they're a member, they'll be notified that they don't have permission to see the forum.
We have our engineering team working to remove the FB Share functionality from private forums. In the meantime, feel free to share content to Facebook, but please know that Share = on is the default setting and this is particularly important for members who post to private forums until this particular issue is addressed on the backend to disable it from private forums.