What's the most popular social media platform for businesses?
Trends and preferences change between both social media users and the platforms' functions and features. Twitter, for example, saw a boost in popularity and users during the Presidential election due to candidates, campaigns, and news outlets posting there both timely and frequently. With the introduction of Facebook Live, there was a rush from users eager to start broadcasting live video from their accounts. From a business perspective, these changes cannot be accurately anticipated, so treating them all as one large map of targets keeps your marketing efforts in front of the crowd regardless of where the users are moving around.
And when taking into account the unknown state of the future, who knows where Google+ will be a few years from now? Should it become the dominant platform and most popular among the others, would you rather be starting fresh or going in with an established internet property?