From Google’s perspective, Google is not a social network meant to compete with Facebook. Rather, it’s an identity system that follows you everywhere.

Think about it: what is more valuable? Inane chatter, memes, and baby photos, or every single activity you do online (and increasingly offline)? Google is about unifying all of Google’s services under a single log-in which can be tracked across the Internet on every site that serves Google ads, uses Google sign-in, or utilizes Google analytics.

Ben Thompson, The Tragic Beauty of Google. Emphasis in the original.