When Facebook acquired Instagram in the spring of 2012, the photo-sharing app had rougly 50 million users and wasn't making any money. The fact that Mark Zuckerberg decided to shell out $1 billion for the rising competitor was greeted with skepticism by some, while others considered it a smart move to step in before Instagram really took off. In hindsight, the latter were right as the chart below illustrates.
The photo-sharing app reached 800 million users in September 2017, its advertising business hit the ground running and Instagram Stories has become Facebook's first line of defense against Snapchat. One can only imagine how much Instagram would be worth today, but considering that (the much smaller) Snapchat’s parent company is valued at nearly $18 billion, one can assume that Instagram’s acquisition price was a billion well spent.