Facebook didn’t have any adverts before it had them. Neither did google, or for that matter cable TV.
Unless you have inside knowledge about Instagram’s business plans, it is reasonable to assume they would have had ads sooner or later, as no other monetization scheme seems sustainable for this kind of “product”.
The nipping of competition in the bud does not make it any less anti competitive.
I was under the impression that it didn't in the early days in the US. For sure, in several other countries, it started without ads, and some channels stayed ad-free whereas others did get ads and getting more of them (though, nothing quite as horrible as what the US has these days -- on a recent visit to New York, my kids tried to watch Cartoon Network on the Hotel Television, and it was probably 30% ads -- never seen anything remotely close to that in UK, Germany and other countries I visit)
One of the early nationally available cable tv channels was TBS. They always had ads. Besides the original purpose of cable was to transmit network TV in remote areas.