We're not sure if this is 100% rolled out yet, but many users are now seeing that share box:
Even if you're not seeing it on the home page, you've likely seen it before, particularly if you're a Google+ user. You'll see it across numerous products, such as Google Docs, Gmail and Google Reader. It's still curiously missing from YouTube.
It's been very interesting to see how carefully Google has been implementing Google+ functionality into YouTube. YouTube has such a tremendous reach, the company certainly doesn't want to alienate those who aren't into Google+. Still, we've seen some subtle things creep in here and there. If you ask me, heavy Google+ integration into YouTube could be one of the biggest things the company could do for its young social network.
This new home page integration certainly can't hurt either. I wonder how long until we see a search box built into Chrome.