They would've had competition.

Whatsapp has other competitors currently. Telegram and Viber come readily to mind, and I am certain there are other services that I'm not aware of, the old fart that I am.

Whatsapp was incredibly popular in most of the underdeveloped world, where it became the de facto messaging app because of the low data requirements and ubiquitousness (and probable subsidies via the ISP provider). This was prior to FB buying them. Out of my group of contacts, the Telegram app notified me over the past year that roughly 2 dozen have signed up for it, e.g. That's another (albeit anecdotal) sign.

I understand the antagonism toward FB/Whatsapp/Instagram, given the consolidation of social networks within the purview of one entity, but not a single one of those services is a monopoly.

Could very well be I don't understand the intricacies of the law, and such consolidation is perhaps illegal for the betterment of society, but I'm not convinced that's the case still. Microsoft, for example, was accused of monopolistic behavior because they had enough power to pressure manufacturers to bundle IE with the OS. No such thing is happening in this situation. IANAL, but that's my simplistic take on it.

I stand to be corrected.

Instagram was not the "main game in town" when Facebook acquired them. It was one of many photo sharing/editing apps and had far less market penetration than it does today. Why should Facebook be punished for acquiring a promising app and turning it into what it is today by integrating it into its ecosystem?

Hindsight is 20/20 but at the time of acquisition it was far from a sure thing and it's safe to say that without Facebook resources Instagram would not be what it is today.

I remember when Instagram was acquired and it seemed like everybody was using them, they were getting mocked on the night TV and SNL, it certainly seems to me they had the primary mindshare and threat to the social media experience at the time.