I have basically zero imagination about big dollar numbers, but I just don't see what they are going to do with $40,000,000 that would generate more than $40,000,000 in value.
Without that context, I have no way of seeing this as anything other than a "bubble" investment, even in a pretty awesome product/team.
It's easier than emit obligation on the market, I guess.
See it as a long-term shot. No doubt that 40M in recruitment will pay if the SO team continue to be half as good as they seems to be today (which is one of the challenges).
They are hiring lots of people to provide new features and keep growing. To add $40m value you've got to add of the order of 4m users which seems quite doable.
Without that context, I have no way of seeing this as anything other than a "bubble" investment, even in a pretty awesome product/team.