Facebook could acquire 100 more Instagrams and have every single one of them fail - or even shut them down - without all that much dilution.
(Cisco and EMC from 1990-2010 are successful examples of this strategy. See DataDomain, Isilon, Cerent, and arguably Crescendo for example acquisitions.)
Exhibit A: https://techcrunch.com/2017/10/16/facebook-acquires-anonymou...
Exhibit B: https://www.engadget.com/2018/07/02/facebook-shuts-down-tbh-...
Facebook knows the lifecycle of social media networks, and knows that the way to stay in front of the inevitable decay is to watch like a hawk for the New Thing and either buy it (Instagram, WhatsApp) or clone it and stomp it with network effects (Snapchat).
The complicated bit is heading off anti-trust regulators.
Same with vine and videos on ig