On another plan (not really related to the argument- but more as to what really made IG stand out), when almost identical services as you mentioned are similarly growing, the only difference is the endorsement behind closed doors. Instagram was endorsed by Adam D'angelo, Jack dorsey and Sequoia w $50M in its war chest
Also, FB itself wasn’t what it is now at the time either.
This information asymmetry is a big issue. It means big platforms can detect competitors early and know when to offer generous acquisition deals to neutralise them. The people working in these competitors don't have this comparative basis to tell how well they're doing.
As a small player, it's very hard to see how successful your competitors are. For example, Google and Apple only provide vague statistics about number of downloads in their appstores, rounded very loosely. But they internally can track detailed metrics. Facebook and Twitter are used as distribution mechanisms, for login, and as sources of contact info. They can glean insights from that.
Facebook has even better insights through the Onavo VPN, which as it tunnels traffic from other apps, can even show which features are doing well https://techcrunch.com/2018/02/12/facebook-starts-pushing-it...
I don't know if a regulator would have had the foresight to see Instagram, which was still as much known for its filters as its social network as anything but a compliment? Look at how Instagram was seen as one of many similar products at the time:
https://www.engadget.com/2012/05/25/instacrt-puts-all-other-...
https://lifehacker.com/5970648/five-best-photo-filter-apps
An early rival, Hipstamatic, was named the Apple App of the Year in 2010: https://techcrunch.com/2010/12/09/apple-top-apps-2010/
Insta has taken on a life of its own over the last 2-3 years, but if they had chose to go their own way, whose to say Facebook wouldn't have clobbered them, in much the way they nuked Foursquare.
I'm not opposed to regulation, per se, but we should also be careful not to indulge in revisionist history.