well also... when you _add_ a feature, it's very hard to discern actual growth. people could've been copying links often, and you don't know. once you have a button, you know. it could end up being the same number of people. fun with bias!
my bet would be low, 1% to 8%, based on similar experiments i've done and seen with sharing over the years. share button is a must-have utility function but strangely it's not an actual growth driver on its own, in my experience. especially if all it does is "copy link to your clipboard".
very nice. send-a-post (via messaging) was pretty powerful on tumblr. save-as-image... i dunno. really cool, nonetheless. i was gonna suggest "make sure the bluesky logo is on there" but you probably have that already.