I don't think many investors are familiar with Oculus VR. To them, facebook has just paid $2B for a company that hasn't released a product to consumers yet, and intends to make one of those headsets that were a big flop in the 1990s.
The other side of it (as you point out) is that institutional investors are generally not in it for long-term gains. If you manage a portfolio at an investment company, are hoping to get a nice bonus this year/quarter, hearing Mark Zuckerberg say he's bought a company that might take 5 - 10 years before it adds anything to Facebook is probably not very impressive.