One thing is that the Rift really
isn't going to match well with Facebook the highly-mass-market product for a long time. I think the DK1's modest GPU demands have given people a misleading impression of what CV1 will actually cost to run. If Oculus takes the high road and ships a 2560×1440@90Hz CV1 before the end of 2014 then the total buy-in cost at launch could be $800-$900 even for most people who already have their own gaming PC handy: you'd need a powerful and power-hungry GPU, and no currently-existing SLI/Crossfire setup is going to cut it either.
With such uncertain prospects for going it alone it seems clear why Oculus would want to be bought by Facebook; like a typical FB acquisition, what's less clear is why FB would want to buy Oculus, since the Rift will have comparatively few owners at first even if all goes well. Three possibilities:
1) Zuckerberg is worried that Oculus and all the other VR efforts could fail, leaving FB without a Next Big Thing to evolve into in future years as VR becomes affordable
2) Zuckerberg is worried that Oculus could fail while Project Morpheus succeeds, leaving VR a failure on the PC but successful on consoles. Sony would love to have its own hit social network, or to make Facebook grovel to gain access to the PS4 platform; Facebook would not enjoy it so much
3) Zuckerberg is another starry-eyed Oculus-loving geek, but unlike all the others he has the spare change to make sure that the VR dream stays alive
My hunch is that it's some combination of all three. Other people are worried about FB wanting to exert proprietary control over the Rift "platform"; I'm less worried about that. If the Rift is a hit on the PC it will have competitors, which will put manners on Facebook and/or just give people somewhere else to go. If Oculus had gone it alone and failed, likely there would be no successful competitors for several years either, and FB logins would be the least of our concerns.