Not sure why people keep comparing these apps. Snapchat is a much more person to person app whereas Instagram is a person to world app. Not a surprise that money would go into instagram.
Then came Instagram and ate their lunch. Now they can't go back because they have already IPO'd with the ad-driven business model (instead of somehow capitalizing on the ephemeral nature of the medium, which would have been much harder for FB/Instagram to copy).
So yeah, this is why people keep comparing these two.