Vas majority of the websites that use font awesome icons are cheaply made projects that fail to get traction most of the time and have a relatively small user base. So using font awesome icons with an idea that it will help users recognize what they represent, is flawed. I doubt regular users who according to you "would have trouble figuring out what the icon means if they did not see it before" even visit the type of sites that use font awesome icons, they are primerely used by small startups or one men shops with users mostly coming from sites like HN or producthunt anyway.