Either. Both languages can accomplish the same thing. The choice really comes down to the framework, not the language: Rails or Laravel/Syfmony/Yii/Cake. But even then a social network is still easily implemented in Rails or any PHP framework, so at that point it comes down to a matter of preference. Try Ruby/Rails, and try PHP/Laravel (Laravel is by far the easiest, most intuitive full-featured framework in PHP), and see what you like.
From a pure technical point of view, there are no meaningful pros/cons to either for the problem you've presented.
That said, let's say you wanted the network to be fully real-time, with lots of notification messages appearing without having to refresh the page etc. For that, you may want to consider using Node.js and Express.js with SocketIO. You can get the same functionality in PHP/Ruby, but not as easily as in JavaScript running on Node.js. Node.js is a better choice for stateful, realtime applications.