It’s no secret. When engineers built Google+, they constructed an SEO juggernaut to dominate search results above all other social platforms. Although Facebook and Twitter are essential to marketing efforts, both restrict Google from accessing much of their data. This limits their SEO effectiveness.
Not so with Google+.
Here’s an experiment: If you use Google+, perform a search for your name and check the domain distribution of first 100 results. The graph below shows what happens when searching my own name.