Also, Facebook doesn't have a monopoly on VR - there are several competitors in the market, like HTC and Valve.

Yeah, and that might be because Valve is too expensive for them to buy.

Valce is also probably not for sale.

There's also a bunch of social features embedded in the Steam client; I wouldn't be surprised if regulators took exception to any acquisition by another company with a social network.

Seems a bit short-sighted because if Oculus becomes the dominant VR platform that Facebook envisions it to be then it's going to dwarf instagram in terms of economic importance (for advertisers and users) over the long term.

That's a big if, one that hinges on way too many things to be able to successfully argue in court about:

1) VR ever becoming a huge player. It might. It might not. Right now there's no real evidence that VR will actually become a big deal, and Facebook could argue that this is a niche offering that can't be anti-competitive because there's no real market yet.

2) The feasibility of VR technology independent of major vendors. There's a strong possibility that without a huge R&D budget VR will never overcome the hurdles that might keep it out of the mainstream. Facebook could argue that removing Oculus would doom VR to failure in the market.

3) The Sirus/XM merger removed 100% of satellite radio competition (there's only one vendor left after the merger), but both Sirius and XM successfully argued that the market isn't big enough to support multiple vendors. Facebook could argue a similar case, that for the health of the VR market there should be fewer competitors.

I'm not a lawyer so I don't know if any of those would be winning arguments, but based on previous anti-trust cases I've followed that would be the strategy I would expect. And it might be why the FTC didn't go down that path.

The flip side of it is that VR isn't mature enough for a company like Oculus to succeed independently. Having the resources of Facebook behind it will help the VR market as a whole (for now).

I think the FTC will deal with that as it comes. Including that in this case would be speculating the future of the Oculus business.