A lot of people here, from to , and more have noticed the number of people adding to them in circles has caught up to or exceeded their followers on Twitter, despite Google+ being out for just under a month, while Twitter just celebrated its 5th anniversary. Many of us joined Twitter 3-4 years ago. of AllThingsD has an analytical post today on just why she and her connections think the counts are the way they are.
My two cents: Adding people to Circles is fun and simple, and you know you can organize people who you might view less often, while with Twitter you put everybody in the same bucket, so you pay a tax for connecting. Also, the engagement here is such that we early users are enthusiastic about following the path forged by other social networks, and finally getting the equation right.
(See: Google+ Really Has the Hang of the Follower Count Game)
/via My Google+ Profile.