Weak engagement
According to a report from We Are Social, The state of Google+, the monthly active Google+ users is only 35%. Google+ has seen extraordinary growth between Q4 2012 and Q4 2013--from 359 million users to 1.15 billion. From a membership standpoint, the numbers are amazing. Currently a mere 35% of registered users are active on a monthly basis.
Thinking forward
Engagement with the network is a clear problem. Still, it's wise to not cast aside Google+ just because of weak engagement numbers. Google+ acts as a glue between Google's products. Binding together these products has tremendous value, and perhaps the association of Google+ as "trying to be" Facebook is narrow sighted. Google+ must be different from it's larger rival--especially in a time when Facebook's cool factor is fading. Google is still drafting the contours of it's social network and that 35% monthly active user base is nothing to balk at.
SOURCE: infographic: We Are Social - wearesocial.net