Official announcement here:
There's also an icon (between "Follow" and the chevron-down menu) in the header of conversations.
It is a great idea to make sharing to other places more obvious - but the icon choice is a bit confusing to me. I think "box with an upwards/outwards arrow" is an iOS-specific convention for sharing. As a web and Android user, this icon is mostly meaningless to me, and I would have preferred a "three connected dots" icon or the more generic "swung arrow pointing to the right" icon.