Part of Twitter's challenge is that it's mainstream, but in a "write-only" way. Every celebrity, politician, religious leader, and journalist may be tweeting 24-7, but their followers aren't engaging in the same way they do on Facebook. A lot of users don't login, tweet, or spend much time on the site/app according to various reports.
G+ is a staggering number of peoole. 2.6 billion last I checked. Its simply that mist of them don't want to be there and say nothing. Engagement is by a fraction of a percent of the whole.
Chris Sacca said a lot about this recently. http://lowercasecapital.com/2015/06/03/what-twitter-can-be-2...