As part of the Mountain Lion refinements and the unification of Mac OS and iOS, Safari received a new “Share” button, just to the left of the address field. It displays a menu of options, including Twitter, Message (the desktop version of iMessage), and Email this Page. Those all make sense, as they are all methods by which a webpage may be shared with other people.
I first discovered this button when I was trying to bookmark a page. There used to be a “+” button that would allow you to add a bookmark, but that was gone. I couldn’t find a bookmark button anywhere, but I noticed the Share button and clicked it. Somewhat unintuitively, the first two options in its menu are Add to Reading List and Add Bookmark. These aren’t methods of sharing at all; they are means by which to save a page to view later. To be perfectly fair, the same icon presents the same menu containing the same options in Mobile Safari, so it is now consistent across platforms. It’s easily learnable, so I don’t consider it to be problematic, and I can’t think of a better label for the button. It’s a sensible combination of similar functions under one UI control—simplification through categorization.