Facebook's inline video player uses it. On linux, I get a popup saying that I must install Flash.

In Safari on OS X, Facebook does use the native <video> player.

I'm guessing that Facebook encodes video h264 which isn't natively supported in Firefox; rather it relies on support in the operating system. I'm not sure if Chrome on Linux supports h264, however since Chrome also includes its own Flash player I guess that Facebook may be using their own flash player anyway.

Pretty sure there are some settings you need to enable in about:config to get H264 working in Firefox (assuming you have the right gstreamer stuff installed).

Firefox can also install the free Cisco h.264 codec.

FB is using it a lot. For videos, for uploading pictures.

BBC iPlayer is flash-only

Not just iPlayer - the main BBC website - including news/weather/sport - requires Flash to display all video content on desktops.

Porn sites.

All my frequent porn sites support HTML5 video now

Good question. There were some I had to use Chrome to view (espn, cnn, facebook-- somebody) and I was quite surprised.

twitch.tv

You can play streams with: livestreamer

http://docs.livestreamer.io/

And chat trough their irc server.

I don't think that's needed, i'm looking at Twitch right now (via the normal way) - and i don't have Flash installed.

On an iOS or Apple device?

OS X, Yosemite, Safari

HBO Go