The first objection I'm afraid I can't really buy - Most of the UK population uses Facebook. Most Mumsnetters use facebook. It's a bit mad to object to facebook on principle - it's like objecting to people.
This is just daft and is irrelevant whether you buy it or not. Just because "most of ths population" use FB doesn't mean everyone on MN is going to like it. It is like saying you don't buy that some people won't eat meat as the majority of the population are not vegetarian/vegan. And it is nothing like objecting to people 
The second objection I just think is misguided, as I've explained. Users' privacy is no more compromised by this button than it ever was.
It might not be "no more compromised" than before but the point is it is much more visable now and before people might not have been aware of the link button but now they are and they don't like it.
So should we change something because some people on this thread dislike it even if we think they are mistaken to dislike it? Even though what they are objecting to is already in place?
You may think we are mistaken not to like it but we are part of MN and without posters you just have a website with adverts no one sees. And the fact that it is already in place is not the issue. Before lots of posters weren't aware of it so couldn't object.
I've been doing this long enough to know that new things are never well-received - but it doesn't always mean they are bad.
That comes across as patronising and lots of posters are telling you this is a "bad" idea and lots of people don't like it.
For me this is a huge loss as I feel unable to discuss something I would have appreciated help with and that makes me feel sad. And namechanging doesn't work for me as someone always guesses who I am and outs me.