It's not like they bought a consumable that's used up, and thus can't buy other necessary consumables.
And a lot of that Facebook value only exists because the universe of owners (and potential owners) believe that Facebook will keep preserving and growing that value by doing these sorts of things.
If that same 'investment' could have had a better 'impact' for the Facebook owners elsewhere, they'd do it.
If you mean some other sort of 'impact', for the good of people other than the Facebook owners, then essentially, no, that investment value can't be alternatively deployed that way. The value exists because of the faith it will be used in self-propagating ways. The equity value would simply evaporate once the confidence in it is broken: it's not transferred anywhere, it just disappears.
And if you engineered a world where this sort of 'wrong' bonanza can't happen, by design, then the big stacks of value either on the Facebook side ($1B), or on the Instagram investors' side ($250K), would never have condensed in the first place. Their existence was conjured up from previous bonanzas, or the expectation of potential bonanzas. Eliminate the potential for such bonanzas and you don't free up value for impactful redeployment elsewhere: the value never even exists.
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.
I wouldn't propose a system in which investments couldn't propagate future opportunities. I think they should at the very least be regulated. But my concern goes deeper into the underlying system and culture in which all of this takes place.
Facebook and Instagram didn't do anything wrong within the context which they operate. But I believe there is something wrong with the context.