Django is good for faster building of full web applications (because lot of stuff is pre-packaged). But, then, that makes it heavier and you may have less control and
you may have lot of unnecessary packaged code shipped.
Considering speed as a factor, from a business (though maybe short-term) perspective, Django could be seen as attractive.
Twilio tech team shares how they a using Flask on their official blog. An excerpt:
Flask-RESTful is a simple, easy to use Flask extension that helps you construct APIs. It gives you a clean interface for easily parsing arguments to your resources, formatting/serializing your output, and organizing your routing. It abstracts away a lot of the HTTP boilerplate code for you. Flask-RESTful does this while staying out of the way of your business logic. It doesn’t prescribe a particular ORM, and it doesn’t have any dependencies other than Flask itself. The goal of Flask-RESTful is to give you a simple and extensible starting point for building your API just the way you want it.
Flask will give more control but then that means – you need to be an advanced coder to achieve that. This also means, from a learning perspective, Flask would given more opportunity (long route) to learn advanced stuff. This also makes Flask good for long term Business because you get control.
Pinterest is one the biggest projects using Flask.