Ok, great, it's nice, big, great...

Now... Instagram knows how to make money. FB knows how to make money. Twitter knows.
They also know how to get users' attention.
They know how to build addictive products.

Those are facts. They know how to do it.

Now... Why would they addoprt third-party's coin?
If they need a coin, they are going to make their own coin.

Let's imagine, Instagram takes Steem, I don't know why, but they do.
What is their benefit to do it?
Maybe, only maybe, their user count would rise a bit.
On the other side, very bad content will be upvoted, selfvoting, bidbots...
They would ruin the principle 1 man 1 like
And users would become frustrated and discouraged. So they will actually lose a lot of users.

Decentralized... Nobody cares. People only care if the final product is good or not. What is behind the courtain, people don't care.
Don't trust me, take a paper and pencil, hit the streets and ask 1000 people