Ahh where to start.
So first things first. One company buying another one is nothing evil per se. It's the goal of most IT startups.
Now, the reason why Google and Facebook buy companies has nothing to do with competition. Both are very aware of the biggest threat of IT companies, which is what basically happened to Microsoft: running stale.
A few years ago, Google had a very specific reputation regarding acquisitions: They give you an offer. A very good offer. They don't negotiate, and if you turn it down, they never approach you again, then that door is closed forever.
That worked quite well for them, until Facebook started screwing them over. Because Facebook was willing to negotiate, and Facebook was willing to pay more if they really, really wanted something. So Google couldn't just do their godfather thing any more, and that's why the prices for those successful startups exploded so rapidly in the last years.
Reality is, Oculus VR in this shape and form will never become a big thing.
I bet you, 10 years ago you would have laughed at the idea of smartphones. They are more expensive than normal cells, they are bulkier, they need to be recharged daily.
I can't tell you if it will become the next big thing. No one can. Everyone who claims he knows is full of BS.
Zuckerberg doesn't know either, but he thinks the possibility is there. Which is how startup acquisitions work. You have something with the potential to become big. Out of 10 companies, 9 crash, one rises. Investors have no idea which one that is. That's why they put their money into multiple companies.
I think you misjudge the position of Valve completely. They make more than a billion $ revenue per year. Probably even a lot more. You don't just buy a company like that. Valve's value was estimated 4B$ in 2010. And they did grow heavily in the last years. By now, they're worth a multitude of that. And even then, you don't buy a company by it's worth, but you have to add something on top so the owners are actually willing to sell. Server infrastructure for Valve, that's pennies, do you really think they have to rely on facebook and their webservers?
Valve is not a ball, Valve is a player.
Instagram, Whatsapp, Snapchat, you know how much profit they made? Nada. And FB paid/was willing to pay billions for them. All of FBs acquisitions have one thing in common: They are not fully developed yet. There was potential. They made no or little money.
OR fits facebooks portfolio perfectly. Valve doesn't.