Preventing Instagram from developing into something that has a negative effect on Facebook. It's a "keep your enemies closer" move.
Now to put the question in context if you have a small company that is worth, say $500,000, then 1% is $5000 you might spend that much (in cash alone yearly) as "insurance", say for property/casualty or liability. And you would pay that no matter what the business climate if you perceived a risk to your business, right?
But it could indicate a bubble simply because if the market is up people are more likely to overpay for anything because they feel very upbeat and enthusiastic about the future.
So paying a large number (and 1 billion is a large number and not trivial no matter how you slice it) would be more likely to happen in a bubble.
But there are to many variables in this that are not know to draw a definitive conclusion.
That said my feeling is we are in a period of irrational exuberance.
Whatever value Instagram has for Facebook, it probably has at least double that value for Google, who could buy it just as easily. By paying a premium and showing major interest first, Facebook preempted an opportunity for Google to get some traction in the social space. Very smart move.
For a fraction[1] of that they could have cloned the software pixel-by-pixel and weaved it in with facebook in a way that the original instagram couldn't.
I have trouble believing instagram was about to turn into an 1 billion dollar threat.
[1] Understatement of the month