We never got to see what would have happened to Instagram, had Facebook gone on to compete with it over time with a cloned product (or perhaps some other purchased competitor that might have sprung up to take on Instagram).
One of the most critical powers of a monopoly (with the typical profits that go with such), is its ability to acquire the smaller competition - frequently at high prices - that threatens it. Which is why Facebook was willing to pay such a seemingly immense sum for WhatsApp (a product with essentially zero sales). And it's why they would have happily paid a lot more for Instagram (another product with no sales) if they had demonstrated continued growth.
Again, you're saying that "big" competitors have advantages, which no one sane disagrees with. Nonetheless that's not what antitrust law is about, and whinging about "monopoly" as a synonym for "big" just leads to pointless arguments like this one.
Also, I said that tech markets tend to reach (quickly) a monopoly situations. But even if it's very fast, it does not mean it's instant. Facebook, Twitter, Instagram, Flickr were all created in the same 10-year time span.
Once more, though: how do you take that frame and then explain the success of Instagram, which launched straight into the face of the unbreakable monopoly you posit?