How to find Google+ers who are interested in a topic and actually engage in it?
[Original idea by: +Graeme McRae]
Here are the instructions (read more below) and a case study on "Google Authorship" topic.

What's your usecase? Have you done it before? You can either refer it in the comments or share yours with these hashtags:  #Engagement , #Circloscope . 

[Notice: if you've been notified about this post, it means that you've been included in this shared circle #publicsharedcircles ]

Instructions:
1- Google your topics and find some top-notch content (on a website, a blog, Google+, etc)
2- Write down the URL address of the posts you found in step 1
3- Install www.circloscope.com on your Chrome browser
4- Click on the circloscope icon in your toolbar to open the extension
5- Choose your personal/page account
6- Load > Data > Ripples > paste one of the URLs from step 2
7- Wait for the process to finish. If you want to add other URLs as well, repeat step 6
8- Now you have a list of all Google+ users who have engaged in those posts by sharing/+1ing/commenting.
9- Press "shift+s" to select all.
9-1- (optional): instead of selecting all, you can use various filters on the left.
9-2- (optional): Example: you may want to narrow down to ACTIVE users in the last month by "Filter > Inactivity > set 'Max' to 30 days"
9-3- (optional): Example: you may want to narrow down to those who are NOT in your circles yet by "Filter > Copies > set 'Min' and 'Max' to 0"
9-3- (optional): Example: you may want to narrow down to those who are complete STRANGERS to you (i.e. neither following you, nor in your circles) by "Filter > Relationship > Stranger"
9-4- (optional): Example: you may want to narrow down to those who have engaged with resharing by "Filter > Engagement > Resharer"
10- Now you can do some action on the selected profiles
10-1- Example: "Action > Circle Operations > Add to a circle"
10-2- Example: "Action > Export selected > to CSV" to segment them with your own tools

Case study on "Google Authorship" topic:
I googled the term and found these posts:
http://www.forbes.com/sites/brentgleeson/2013/09/30/everything-you-need-to-know-about-google-authorship/
by +Brent Gleeson
http://www.seroundtable.com/google-authorship-project-closing-17581.html
by +Barry Schwartz
http://www.huffingtonpost.com/jayson-demers/the-complete-guide_b_2831249.html
by +Jayson DeMers
http://searchengineland.com/troubleshooting-google-authorship-issues-174903
by +Janet Driscoll Miller
http://www.martinshervington.com/everything-you-need-to-know-about-authorship-author-rank-and-social-seo-with-mark-traphagen/
by +martin shervington
http://blog.bufferapp.com/why-google-authorship-is-so-important-for-the-content-you-create-and-how-to-set-it-up
by +Kevan Lee
http://moz.com/blog/google-authorship-troubleshooting-article-attributed-to-wrong-author
by +Mark Traphagen

The shared circle of this post conatins the "Google Authorship" engagers who:
- have RESHARED this topic,
- have been ACTIVE in the last WEEK
- are completely STRANGER to me (i.e. neither following me, nor in my circles)
along with the authors of the above posts, and myself :D