Wrong. The Instagram acquisition was largely defensive, due to Instagram dominating a space (mobile photos) in a way that would've eventually hurt Facebook. Facebook snatched them up early to prevent a huge competitor, and $1B was the price they had to pay. Nobody can argue that the team or technology was worth $1B, and Facebook will not see a big return on their investment - at least not anytime soon. Instagram doesn't even have an established revenue model.
Even if the purchase was 'defensive' – preventing s competitor from using Instagram to chisel away at Facebook's monopoly – they expect the result of the deal, versus the alternative, to be worth even more to them than the $1B they gave up. (This is essentially tautological, unless you think Zuckerberg is irrational.)
My guess is that the "something more valuable to them" is protecting their $100 billion valuation. It all makes perfect sense if you are prepared to suspend reality and accept that $100 billion is what Facebook is worth.