That's not true; Zuckerberg spent a lot of time and resources pursuing HTML5 Facebook app which was meant for all smartphone devices. When he realized HTML5 web app it's not going to work [0] Instagram already emerged and started growing rapidly. That's when he figured out he needs to make native mobile Facebook app and purchase Instagram in order to gain foot in mobile space and in order to eliminate possible competition in social photo industry.
[0] https://techcrunch.com/2012/09/11/mark-zuckerberg-our-bigges...