Acquisition value != societal value.
Instagram wasn't worth $1 billion to society, it was worth $1 billion to Facebook, which works out to $30/user. Facebook can reliably turn users into gold, therefore it was a reasonable purchase for them.
It's analagous to someone saying, "Academic researchers should be paid more because of what they're progressing mankind". But they're typically not paid more than those working in a for-profit organization who is also doing research.
Instagram was worth that sum to Facebook, just as Lucasarts is worth that sum to Disney. Neither are worth that much for society, though Lucasarts happen to contribute more to society over it's 40 years of existence.
That's the part you need to justify, since that's the only part that has anything to do with a supposed bubble.
Facebook said it themselves, it's less about '$1bn' and more about '1%'.
Besides, usually when people are flinging around that ill-defined loaded term, they're comparing something like a farmer to a social media platform, where one provides obvious concrete value and the other is providing value much higher up the Maslow hierarchy. But here we're talking about two entertainment companies, one built on two-way communication and one built on the older one-way consumption model, that peculiarly 20th-century aberration. Is it really so obvious that the valuations are that wrong? I'm totally unconvinced it is so obvious that it goes without saying.