A big part of anti-trust law in the US is that natural monopolies are permitted - you can build a business that dominates your market segment and not be subject to anti-trust proceedings, but you cannot use that natural monopoly to advantage your other lines of business by providing them with exclusive, preferred or other special treatment in said natural monopoly. This was the crux of US vs. MSFT.
The argument here is that Facebook is a natural monopoly and by only allowing Instagram to post automatically to a users timeline, they are leveraging that legal natural monopolies power to advantage Instagram.