It’s quick and easy – in fact, if they’ve already signed in to the platform in question, it involves one, or at the most two, clicks. In itself, this isn’t a new idea. We’ve been signing up to sites with our Facebook details for a while now, and even before that, Windows Live / Microsoft ID sign-in was commonly used. Now these two have been joined by Twitter, and also LinkedIn, meaning that almost the entire online population has the option to use social sign-in if they choose to.