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.