AFAIK Twitter allows search indexing and is already a publisher on their network.
Google will "enhance the tweets" by showing 4-5 "relevant ads" before you even see the tweet. Tweethancer
Also, imo, calling them "similarly sized" would be incorrect, except perhaps in signed up users (from which google got a ton from the "login with google and your account is already setup", not daily active users)
https://en.wikipedia.org/wiki/Google%2B#Growth
https://en.wikipedia.org/wiki/Twitter (see sidebar)
> User engagement on Google+ was low compared with its competitors; ComScore estimated that users averaged just 3.3 minutes on the site in January 2012, versus 7.5 hours for Facebook.[22][23]
Hell, the Chromecast screensaver photos are hosted on Google Plus.
> Google+ currently has low usage and engagement: 90 percent of Google+ user sessions are less than five seconds
The G+ numbers looked suspicious if you ran any sort of public website, where e.g. I saw metrics for referrals or shares at least an order of magnitude lower than Facebook or Twitter.