You can always spot the Twitter users, here, because they add you to their circles even though they don't know you.

Dear Tweeters, hint: When I add you to a circle and you don't know me, you need do nothing. I will not be insulted. Google+ is not!

Conversely, if I don't know you personally, I probably won't add you if you add me (and I won't be sharing too much publicly, either).
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