It's not clear which company you're referring to, but
- Instagram was at one point a startup. It's common for startups to write in a scripting language to optimize for speed of adding features. Then if you grow or get acquired, the scripting language often eventually gets replaced by a language more optimized for maintenance cost, safety, and/or speed.
- Data scientists often only really know scripting languages, and at any rate scripting languages are useful for prototyping algorithms that need to change daily. Hence a lot of hedge funds use Python. For code that is stable and really matters for performance purposes, it's common for funds to use C/C++ or even FPGAs.
- Instagram was at one point a startup. It's common for startups to write in a scripting language to optimize for speed of adding features. Then if you grow or get acquired, the scripting language often eventually gets replaced by a language more optimized for maintenance cost, safety, and/or speed.
- Data scientists often only really know scripting languages, and at any rate scripting languages are useful for prototyping algorithms that need to change daily. Hence a lot of hedge funds use Python. For code that is stable and really matters for performance purposes, it's common for funds to use C/C++ or even FPGAs.