The original deal was "valued" at $1bn. It closed at a value closer to $747 million.
Instagram is doing just fine either way.
The point the parent was making (IMO) was that the terms of the deal haven't changed, the point you are making is that the value of the deal is changing. When the deal was made part stock (variable) and part cash (invariable) the folks at Instagram 'bought in' or agreed to the fact that the value would vary with FB's price so the fact that the stock component is worth half what it was, may not have been 'expected' but it was already accounted for.
The discussion about stock collars (a financial mechanism that keeps the variable bit within some bounds between the time the deal is made and when it closes), suggested the Instagram guys could have made a better deal for themselves, but we don't know, perhaps they (Instagram) felt that $300M was fine even if FB went to 0 (it won't but its an example).