This is a small excerpt from a big post by HubSpot on the value of Google+ for SEO:
The biggest difference between Google+ and every other social network, in terms of SEO value, may be the way that user-created content is treated.
Public posts on Google+ are indexed for search, unlike most tweets, Facebook posts, etc. [Note: Twitter has told Google to not follow most links and Facebook’s privacy settings prevent many posts from being indexed – it’s not simply because Google won’t index content from other platforms]. Sure, your profile on other social networks will probably rank well for name-related searches, but there’s no way to target keywords and long tail phrases with specific social posts.
The impact of indexing Google+ public content goes far beyond your personal posts. It means every time that your content gets shared by someone else, you’re gaining link equity as their posts are indexed.
Quick summary: there are a couple primary SEO benefits of using Google Plus. First, posts get crawled and indexed – and it happens quickly because, among other objectives, Google is trying to use Google+ to enhance real-time search. Secondly, Google Plus is used to optimize search results with rich snippets, including the popular Google Authorship feature that allows the author’s picture and information to appear in search results.
Get the rest here: How To Hack Google Plus For SEO Value.
I’m not saying you should abandon any other strategy that is working for you — I’m just encouraging you to revisit Google+ if you have not done so already…