I find it invasive, because it moves the goalposts.

I knew what I signed up to here, when I signed up to it.
I don't really need reminding of what it was: like I said before it's a forum of charmingly crusty and essentially harmless old folkies - which despite the 'openness' of the internet at large still existed in it's own little quiet corner of the internet, essentially unmolested by all the virtual viral bullshit out there.
If the 'share this thread' remains I think I may well go the way of others of my aquaintence.

I just like to know the score. We each make an educated choice about how much and what kind of information we share within a virtual community like this. And the Share function subtley but intrinsically alters the existing boundaries of this community, as Janie say's.

The fact that I leave my backdoor unlocked for guests, isn't the same as putting up a big sign in my window saying "burgle me!"
It is perhaps a subtle, but important difference.