It's either that or YouTube. I guess you could argue for Apple acquiring NeXT, but that's an acquisition that isn't really able to be measured in the same way.
Why not since Bloomberg is now pulling numbers out of its ass? A billion MAU with projected $10 billion annual revenue gives a 10x annual revenue valuation. Apple has long surpassed a billion iOS units sold, and annual revenue is $230 billion, give or take, so it should be worth $2.3 trillion. Not bad for an inflation adjusted acquisition worth $654 million.
And Apple is not a "fast growing business"? Have you looked into their "Services" category? IG's REVENUE is claimed to be $10 billion. Apple's Services NET SALES was $9.19 billion. In the last QUARTER. So, yes, Apple is a "fast growing business", and is growing faster than Instagram. If Bloomberg was fair, Apple's services alone is a $400 billion company.
Taking a long-term view, IG's revenue comes from advertising i.e. it's a one-trick pony, just like Facebook. Apple is a diversified company; even though iPhone makes up >60% revenue, is anyone with an iota of financial sense going to complain that it's a bad thing?