Hello. And sorry you've had to wait so long for some MNHQ response. That's not ideal. 
We hope we can reassure you and the other regulars on the Step parenting boards about the Facebook button. First off, it's nothing new - it's been possible to link MN threads to Facebook for nearly nine months now, through the FB button in the Viewport bar (the dangly thing that comes down over the top of threads). It's just that now you can see when people have linked the thread to Facebook - because there's a little number counter that keeps tally.
Then, of course, there's the whole issue of MN and privacy. We know it's very easy - because of the tremendously friendly and intimate atmosphere of long-running support MN threads in Topics such as Step parents - to think that what you post on Mumsnet is private and can't be seen by other people.
In fact, Mumsnet is one of the most-searched and most-quoted discussion forums in the UK and (almost) every discussion thread is indexed by Google and instantly accessible to anyone who types the right keywords into Google Search. We always caution everyone not to post anything on Mumsnet that's so identifying and personal, they wouldn't be comfortable with other people they know finding and reading it.
We completely understand that you and the other Step parent regulars find tremendous release and comfort in posting things that you may not feel able to articulate in 'real life' but we reckon it's always worth bearing in mind that your posts are accessible to anyone with an internet connection - whether there's a Facebook button on it or not.
There are, of course, upsides to this: we're sure there are lots of 'lurkers' who've come across the Step parents topic from elsewhere on MN, or from Google (or even from Facebook!) and who, although they may not post, read and draw huge strength, insights and comfort from what others post. And that's a truly wonderful thing - for which we all at MNHQ owe you, and the others who post about their experiences, huge amounts of thanks.
Perhaps the answer to all this is our topic called Off The Beaten Track? For those of you who don't know already, it's a special topic that we've created to allow people to have discussions that aren't tracked or indexed by Google and which are deleted from our archive after 30 days.
Maybe you could use this as a kind of Step parents offshoot: keeping your main threads in Step parents, as it they are now, for the benefit and comfort of allcomers (and lurkers), but using a special Step parents thread in Off the Beaten Track as a place where you can go, as and when you need to, to vent/chat about the extremely private and personal?
Wishing you all much festive cheer - and even more love and joy in the New Year!