Even though Rift might not be locked to a hardware platform it will likely be tightly tied to the Facebook platform. Welcome to the Metaverse, please login using your Facebook account to proceed.
That's a good point. I think the difference is that instagram had an existing userbase and social ecosystem that complemented Facebook nicely as another content stream.
I don't think you will need a Facebook login to play games, but Facebook will likely monopolize any social aspects. Even if they don't overtly lock out other social players, I think that developers will be skittish to invest a lot of time and money developing the new Second Life with the risk of Facebook cutting them off at any point if they are seen as a competitor.
I think this was mentioned on the ATP podcast, but I can't remember...
Facebook isnt trying to be Apple and buy companies that contribute to their core products. They want to be Disney (who owns ESPN, Pixar and many others).
They've bought these companies (Instagram, Parse, WhatsApp and now Oculus) to largely leave them alone as independant companies operating within Facebook.