In 1.5 years Instagram had 50 million monthly active users[0]. Yea it was no-brainer considering other mobile photo apps existed and Facebook and Twitter were also doing photos.
[0] https://techcrunch.com/wp-content/uploads/2017/04/instagram-...
https://news.ycombinator.com/item?id=3817840 https://news.ycombinator.com/item?id=7266618
Unless you're an extremely successful businessman yourself, calling those acquisitions no-brainers is just completely dishonest.