If i were to build a social app what language is best to learn?

These days the trend is either python stack or nodeJs stack.. one of these 2 should do. 

Node.JS

Node.JS was initially released on May 27, 2009, by Ryan Dahl. Node.JS is the open- source, cross platform that is built on the V8 engine for building fast, scalable and dynamic applications.

Node.JS has a modern concept of single threaded event callback mechanism which makes it awesome and efficient than the traditional threading concept. It gives you the opportunity to connect scripting languages with all the power of network programming.

Python

Python was developed 25 years back in 1991. It is a multi-purpose, highly productive, high-level language for making the highly portable web and mobile applications.

Python has a distinctive multi-programming paradigm support including object-oriented, imperative, functional and procedural styles. It lets you program in fewer lines of code than other object-oriented languages.