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I'm not generally a fan of image replies, but the top comment on reddit really does nail it:

What's the 17 for?

It's a play on iOS notifications. They show a red number in the top right corner to signal how many notifications are unchecked. See: http://img.ehowcdn.com/article-new/ehow/images/a04/pb/9e/rem...

Looks more like Facebook notifications to me:

It doesn't look anything like the notification in that image. It does, however, look exactly like the G+ notification thing.

I remember it floating around when G+ launched.