I remember twitter used to have a retention rate of 30% in 2009 when it was starting to get popular, and it was probably less so before that https://www.google.com/search?q=twitter+retention+rate+&... I used to see lots of twitter dead accounts at that time too.

If we apply this to google+, then it would be 18 million active users if the 60MM is to be believed. That being said, no numbers of active users have been announced so there is no way to know.