It’s hard to see examples of FOSS being used widely between people. This could be the 2nd time after Mastodon that a FOSS project receives massive attention.
I can’t recall other software that the public might have used without knowing it’s open source. The ones I can get off the top of my head may be Firefox or Inkscape.
There’s plenty of open source software that gets a lot of usage, like Audacity, OBS, VSCode, Python, etc.