Most twitch streams are limited to just 3,500kbps, some use up to about 5,000, think you have to be partnered for that though. I'd guess the average bitrate for a partnered stream is like 4,500.

As far as I know facebook's bitrate is far less, like 2500? Might be higher though for this new thing because youtube gaming allows up to like 18,000kbps nmnm

Anyway, just put your bitrate as high as it can go for where you want to stream and use normal x264 encoding, at lower bitrates using your cpu to encode will look much better.

If you have an fps hit that's too much for your pc to handle then use HVENC which is GPU encoding, which will look worse at lower bitrate but give little to no performance impact

So my best guess is the new FB streaming allows for higher bitrate than twitch (if it visually looks better) and uses HVENC to keep the performance hit low. It could also force a constant bitrate which would look smoother and never lose quality

It also depends on what's on the screen, if you're comparing 2 different games one could have way more particle effects and small things flying around which will likely destroy the video and make the video into a laggy mess even at decent bitrate.

But I dunno exactly because lol facebook

tl;dr gpu streaming = no fps lost & more bitrate = better quality