FFMpeg is probably not as up high since video processing only needs to be done on the servers that receive media. I doubt most phones are running FFMpeg on video.
Chrome and Firefox use FFmpeg libraries to decode media, so it's in more places than you might think! (But also, ChatGPT said it's not used in Android browser apps because they would use Android's "native" media stack).
My earlier comment about "SSL" is that the actual library might be OpenSSL, BoringSSL, WolfSSL, GnuTLS, or any one of a number of others. So the number of uses of each one is smaller than the total number of "SSL" uses.