Monopoly? In 2012, Instagram had 40 million users and no revenue[1]. Hardly a monopoly.. Everybody mocked Facebook for buying up Instagram for the amount of money they were offering. Nobody (or, perhaps, very very few amount of people) at the time thought it would become a $100bn company. You really think the government should have had the foresight to block this? Wishful thinking on your part..
I don't think I misunderstand. In 2012, there were quite a few up and coming social media apps. Twitter, Vine (2013), Tumblr, Pinterest, Snapchat (2011), and even Google+. Plenty of competition from both small and big companies. I'm not saying Facebook isn't a monopoly but given the options, you can't possibly think that at the time Instagram was going to be the "Facebook killer".
In hindsight, sure, we'd all love that. But seeing the recent FB hearings with congress, it seems they are still figuring out what the internet is.
[1] https://techcrunch.com/2012/04/13/instagrams-user-count-now-...