According to the Steam Hardware Survey, the HTC Vive is dominating the Oculus Rift by more than a factor of two (0.15% to 0.06%). More-so, its rate of change is also double that of Oculus (0.06% to 0.03%). If these numbers are accurate, this means that the SteamVR is massively overtaking Oculus SDK in terms of both amount and rate of change.
Now the questions are “why?” and “what does that mean?”
The most obvious reason, to me, is that HTC has much better availability than Oculus. For the last month, they announced that the Vive ships within two-to-three business days. If you look at Oculus? The website tells you to expect it in August. It is currently the second day of July. While a month is not too long of a time to wait, it would make sense that a consumer would look at the two options and say “Yeah, the this week one, please.”
If that's the case, then the platform battle could be decided simply by retail availability. It wouldn't be decided by a Valve-developed first-party game. It wouldn't be decided by DRM locking games into an exclusive deal. It would simply be decided by “you can buy this one”. That is, unless Oculus ramps up production soon. At that point, we'll need to look back at hardware surveys (not just Steam's) and see what the split is. They could catch up. They could be left behind. Who knows? It could be another factor altogether.
For now, the Vive seems like it's the crowd favorite.