I usually encode with libx265 and libfdk_aac to get good quality and small file size for posting in Facebook. I use ffmpeg as follows.

ffmpeg -i input.mp4 -vf -c:v libx265 -c:a libfdk_aac output.mp4

If Facebook, YouTube and others video hosting sites re-encode all videos uploaded, is the codecs used still matter?

If I use h.264 codec with mp4 or MOV format as Facebook recommended, will it reduce or bypass the Facebook's video processing?

If I use h.265 for smaller file size, will the viewers of my video save their mobile data usage?