A couple days ago the little notification identifier switched from a number (which Facebook appears to have adopted for unread posts, by the way) to a bell with a rather cute smiley face. Why Mr. Jingles? Because Google calls him that. Blaze through all of your notifications and click on Jingles, and he does a fun little dance.
Jingles has just hit the Android app for Google+ as well. And along with it comes a new button to mark all of your notifications as read. It looks like a little bar graph, and it's now on the web version of Google+, too. Tap it or click it, and your notifications shall bother you no more.
The update should be rolling out now.