I'm having trouble finding functional 3D content on the web.
Poly.google.com has some cool 3D models and scenes, but everything is at my feet and I'm looking down at it from some distance with no way to move the image around.
At Sketchfab everything looks horribly low resolution while it looks good in their demo app that can't connect to their site.
Youtube 3D often puts all four corners of the image in the center of my view in a weird grid that looks like the crosshairs in a scope.
For YouTube, you have to click the "full screen" icon. When the Oculus browser switches to "full screen" mode, there's a floating control that allows you to switch modes. 2D, 360, 180, 360-3D, 180-3D. I have also found that if you click on some of these multiple times it will switch into different "submodes". (e.g. 360-3D over/under versus 360-3D side-by-side) But you may still find videos that are in modes that the Oculus browser doesn't support.
Also, I haven't found a way to pause/play video except in 2D mode. So, I usually pause the video, switch modes until I find the right one, switch back to 2D to hit play, and then switch back to the proper mode.
Also, it looks like sometimes (like the Elton John VR180 videos), YouTube doesn't detect the Go as a device it can send VR180 to, so it just shows you what it would send to a regular browser.