I find it's very easy to imagine how the landscape could change for FB to be broke in five years; Anyone can build a better way to "social network", and anyone will. Guaranteed. The only real question is... what happens if Facebook can't buy them?
I admire what Facebook has achieved, but it's clear to me that they don't understand what their actual role is, or what people "want" from Facebook as a company if it is to survive. There is an air of mild desperations to the whole enterprise that hints at the acknowledgement of an imminent demise. This acquisition reeks of that desperation.
The oculus VR acquisition feels special, in that they aren't just buying users/reach, but something more substantial and forward looking. If they're smart, they'll let the oculus team work on the track they've been on, and let Facebook teams develop the "non-gaming VR experiences" they want to push.
Hard to think how much the landscape would need to shift for FB to be broke in 5 years. If Oculus delivers on their VR dreams, that alone could potentially bring an almost-bankrupt FB back from the brink.