The new feature is called Find my Face, and it helps tag photos of people in pictures, provided they've activated the feature.
Google has obviously learned a lesson from Facebook, which suffered some backlash - even an EU probe - over turning its face recognition feature on by default. In Google+, you can accept or reject someone tagging you or turn the feature on and off and, most importantly, the feature is opt-in.
Find my Face will be rolling out to users over the next couple of days.
Gmail has also been upgraded with a couple of social networking features, making it easier to add people to your Google+ Circles from Gmail and share stuff on Google+ without leaving your inbox.
Furthermore, you can now also filter messages in Gmail according to your Circles; for example, you can see only the messages from your family, work colleagues or any other group of people you've defined as a Circle.
Google will be rolling out these new features to users' Gmail and Gmail Contacts over the next couple of days.