Google's Zach Yeskel announced in a Google+ update that Google+ users who invite others to the service will get notifications when that person joins. That means you'll no longer have to send out invites only to blindly wonder if they accepted your invitation until you see them in your stream.
The notification will tell you "Username has accepted your Google+ invitation."
"Over the past few weeks we’ve added a few features that make it easier to invite your friends to Google+," said Yeskel. "For example, we launched a custom invitation link (for those times when you don’t know someone’s email address). But up until now, it’s been hard to tell when people use your invitations to sign up."
Yesterday, Facebook announced some new sharing controls for users that are quite similar to Google+'s sharing functionality, in that you can better control who sees each post: public, friends or a custom list (with Facebook lists coming soon). It's very much like the Circles way of sharing.
Google noticed some similarities as well, as Google's Bradley Horowitz (VP of Product for Google+) suggested in a talk with Tim O'Reilly soon after Facebook's announcement:
If you've got a bit of time, the discussion above is worth listening to, as they get into pseudonyms and anonymity, brands and more.
One interesting thing he notes is that users will see some "exciting" new things with the share box "literally in days."