Google+ now highlights posts that you +1′d to people in your circles.
In other words, Google is trying to make the Google+ stream more like the Facebook News Feed, which ranks various posts based on its own algorithm. Just like Facebook, Google is promising to respect your privacy by only highlight posts you and your friends would have otherwise been able to see.
In fact, Google notes there are some changes for managing your Circles to help you control who can see your activity, as well as what appears in your main stream. These are necessary for this new Google+ feature to exist while letting you maintain your privacy:
You can now visit your settings to decide who, if anyone, might see your +1s highlighted in the stream.
In all cases, friends will only see +1s and posts they already had permission to see, Google says it is just calling extra attention to them.
If you’re seeing too much activity from a given circle, you can always turn down its volume, or exclude it from Home entirely.
In short, if you want to keep using Google+ with peace of mind, you’ll want to visit these two help pages to change your settings around accordingly:+1 activity updates in the stream and Content that appears on your Home page.